Significant Reduction In Heat Transfer Research Articles

Comparative analysis of structural characteristics and thermal insulation properties of ZrO2 thin films deposited via chemical and physical vapor phase processes

ZrO2 is a flagship material for the development of efficient thermal barrier coatings not only for aerospace applications but also on steel molds for plastic injection, especially to produce high quality miniaturized parts of low environmental impacts. In the present study, ZrO2 thin films of thicknesses ranging between 4 and 40 μm were deposited on steel substrates by two key gas phase technologies, magnetron sputtering (PVD) and direct liquid injection metal organic CVD (DLI-MOCVD). The film morphology, structure and chemical composition were comparatively investigated and correlated to their heat transfer properties. All films were nearly stoichiometric and presented a columnar structure mainly formed of the monoclinic crystalline phase. The magnetron sputtering coatings were denser and smoother than the CVD ones, for which highly porous tree-like microstructures were observed without degradation of their mechanical properties. All films exhibited efficient thermal insulation properties, the thickest 40 µm magnetron sputtering coating displaying the most significant reduction in heat transfer. The CVD films provided the highest thermal gradient decrease across their thickness, probably thanks to their higher porosity associated to a multi-angle tree-like columnar structure, both acting as scattering zones of phonons involved in heat flow diffusion. By controlling the local deposition conditions influencing the nucleation and growth mechanisms, the film porous microstructure can thus be tuned to optimize the coating thermal properties.

Thermodynamic analysis of heat transfer reduction in spark ignition using thermal barrier coatings

This work uses a 0D thermodynamic engine model coupled to a 1D surface temperature solver to study the potential of low thermal inertia thermal barrier coatings (TBCs) on combustion chamber surfaces to increase the efficiency of spark ignition engines. Under ideal conditions, coating the piston crown, head, and valve faces with a TBC with a thermal inertia of 640 J/m2 K s1/2 resulted in less than a 1% relative improvement in efficiency. Despite using a low thermal inertia coating to avoid open cycle charge heating, the reduction in closed cycle heat transfer, which is the pathway to increasing efficiency, increased knock propensity. Therefore, any efficiency gain through closed cycle heat transfer reduction in spark ignition is offset by the need to retard spark timing to counter knock. An exergy analysis of completely blocking heat transfer for a 10 crank-angle degree window showed that on a low compression engine, like those used for stoichiometric gasoline spark ignition, the maximum efficiency gain achievable was limited compared to a higher compression ratio engine. Furthermore, the high gas temperatures of stoichiometric operation mean that even state-of-the-art TBCs cannot elevate surface temperatures enough purely through temperature swing to achieve a significant reduction in heat transfer near top dead center, where work availability is highest. Overall, these results indicate that low thermal inertia TBCs are ill-suited for achieving an efficiency benefit in spark ignition through a heat transfer reduction pathway. Instead spark ignition TBC research should explore ways to use low thermal inertia TBCs to achieve open cycle charge cooling to reduce knock propensity.

Dropwise Condensate Comb for Enhanced Heat Transfer.

Dropwise condensation on superhydrophobic surfaces could potentially enhance heat transfer by droplet spontaneous departure via coalescence-induced jumping. However, an uncontrolled droplet size could lead to a significant reduction of heat transfer by condensation, due to large droplets that resulted in a flooding phenomenon on the surface. Here, we introduced a dropwise condensate comb, which consisted of U-shaped protruding hydrophilic stripes and hierarchical micro-nanostructured superhydrophobic background, for a better control of condensation droplet size and departure processes. The dropwise condensate comb with a wettability-contrast surface structure induced droplet removal by flank contact rather than three-phase line contact. We showed that dropwise condensation in this structure could be controlled by designing the width of the superhydrophobic region and height of the protruding hydrophilic stripes. In comparison with a superhydrophobic surface, the average droplet radius was decreased to 12 μm, and the maximum droplet departure radius was decreased to 189 μm by a dropwise condensate comb with 500 μm width of a superhydrophobic region and 258 μm height of a protruding hydrophilic stripe. By controlling the droplet size and departure on hierarchical micro-nanostructured superhydrophobic surfaces, it was experimentally demonstrated that both the heat transfer coefficient and heat flux could be enhanced significantly. Moreover, the dropwise condensate comb showed a maximum heat transfer coefficient of 379 kW m-2 K-1 at a low subcooling temperature, which was 85% higher than that of a superhydrophobic surface, and it showed 113% improvement of high heat flux or heat transfer coefficient when it was compared with that of the hierarchical micro-nanostructured superhydrophobic surface at a high subcooling temperature of ∼10.6 K. This work could potentially transform the design and fabrication space for high-performance heat transfer devices by spatial control of condensation droplet size and departure processes.

Effects of Hall Current and Viscous Dissipation on Bioconvection Transport of Nanofluid over a Rotating Disk with Motile Microorganisms.

The study of rotating-disk heat-flow problems is relevant to computer storage devices, rotating machineries, heat-storage devices, MHD rotators, lubrication, and food-processing devices. Therefore, this study investigated the effects of a Hall current and motile microorganisms on nanofluid flow generated by the spinning of a disk under multiple slip and thermal radiation conditions. The Buongiorno model of a nonhomogeneous nanofluid under Brownian diffusion and thermophoresis was applied. Using the Taylor series, the effect of Resseland radiation was linearized and included in the energy equation. By implementing the appropriate transformations, the governing partial differential equations (PDEs) were simplified into a two-point ordinary boundary value problem. The classical Runge-Kutta dependent shooting method was used to find the numerical solutions, which were validated using the data available in the literature. The velocity, motile microorganism distribution, temperature, and concentration of nanoparticles were plotted and comprehensively analyzed. Moreover, the density number, Sherwood number, shear stresses, and Nusselt number were calculated. The radial and tangential velocity declined with varying values of magnetic numbers, while the concentration of nanoparticles, motile microorganism distribution, and temperature increased. There was a significant reduction in heat transfer, velocities, and motile microorganism distribution under the multiple slip conditions. The Hall current magnified the velocities and reduced the heat transfer. Thermal radiation improved the Nusselt number, while the thermal slip conditions reduced the Nusselt number.

Numerical analysis of the longitudinal size of the partition on natural convection heat transfer and fluid flow within a differentially heated porous enclosure

AbstractThe article deals with the effect of longitudinal size and shape partition embedded within a differentially heated porous enclosure. The objective is to curtail the heat transfer rate across such porous enclosures by means of partitions embedded within. The partition shapes under consideration are straight vertical left‐inclined, right‐inclined, L‐shaped, wavy, corrugated, and square‐wave. It is sought to find the most effective combination of partition length and shape that could serve the required objective. Also, many times, due to the constructional constraints of the porous enclosure or cavity, using full‐length partitions may not be feasible. In this regard, it is also sought to find the partition length that is to be maintained for achieving a significant reduction in heat transfer without much compromise. The results of the current study are useful for thermal design engineers particularly in the field of thermal insulation, solar heating application, and packed bed energy storage systems where the major challenge is to reduce the heat transfer across the system. The parameters under consideration are the longitudinal length L and Rayleigh number Ra. All the partitions under study are evaluated for bottom‐wall and top‐wall attached conditions. Some of the notable findings are that for smaller‐sized partitions (B < 0.5), L‐shaped partitions are most effective in controlling the convection heat transfer rate across the enclosure while for larger‐sized partitions (L > 0.5), square‐wave‐shaped partitions should be preferred for effective reduction in the rate of convection heat transfer.

Large eddy simulation of separated flow to investigate heat transfer characteristics in an asymmetric diffuser subjected to constant wall heat flux

In the present study, heat transfer characteristics of an asymmetric diffuser with separated flow has been studied. The flow separation is triggered due to wall expansion in two directions. Large Eddy Simulation (LES) approach is adopted to solve the turbulent separated flow and heat transfer in such diffuser at Reynolds number of 10000. For this purpose, a finite volume solver is extended in the OpenFOAM framework to solve the energy equation for incompressible flow. The extended solver has been adjusted to deal with backscatter phenomena and to prevent non-physical heat transfer results and numerical instability. An appropriate grid resolution is employed to perform LES calculations and predict the characteristics of the heat transfer within the separated flow. The numerical results are validated against measurements and Direct Numerical Simulation (DNS) results. The present study showed that the low mean velocity and turbulent kinetic energy (TKE) in a separated flow region are responsible for generating high temperature hot spots resulted from significant reduction of heat transfer from the walls. It has been observed that the heat transfer from the wall is increased slightly before the flow re-attachment region. The applicability of the Reynolds analogy in the separated flow zone for this problem has been examined. Moreover, the analysis of the computational performance showed that increasing the number of computational cells can improve, to certain extent, the convergence rate of the solver and therefor, reduced the computation cost.

Effect of the axial cavity with an opposing high-pressure jet combination in a Mach 6 flow condition

Flow control techniques mitigating the aerothermal heating over bodies are essential for long-duration hypersonic flight. The combination technique of a nose cavity with an opposing jet is exhibited promising effects. There is a lacuna of experimental studies on unsteady aspects of the flow and the heat transfer effects within the cavity. In the current study, we investigated the heat transfer variations and nature of shock around a blunt body with a cylindrical and parabolic cavity geometries combined with opposing jet. Experiments are conducted in the hypersonic shock tunnel for a flow Mach number of 6 for an opposing jet pressure ratio of 18. Time-resolved high-speed schlieren imaging and heat flux measurements over the model surface and within the cavity are carried out. The cavity flow is established by verifying the measured heat flux, pressure, and shock oscillation results. The combination technique shows a significant reduction of heat transfer and flow oscillations in comparison to the nose cavity technique. Notably, the parabolic cavity with the opposing jet showed about a 33% reduction of surface heat flux and a 43% reduction in flow oscillations. The substantial increase of heat flux due to the shock interaction is noted on the nose surface for a cylindrical cavity geometry, and such effect was not prevalent in the case of flow with parabolic cavity geometry.

Modeling of wall condensation in the presence of noncondensable light gas

During a loss of coolant accident in nuclear reactor, significant amounts of steam and hydrogen can be released in the containment. Condensation of steam in the presence of noncondensable gases on the containment walls and structures is a key issue because of its role in removing heat from the atmosphere. Extensive experimental and theoretical studies have been carried out for a better understanding of this complex phenomenon which involves several physical processes and parameters. When condensation takes place in the presence of noncondensable gases a liquid film is formed and noncondensable gases accumulate at the interface. The diffusion of steam through the gaseous layer depends on the gas composition, velocity, temperature and pressure. The formation of a gaseous layer leads to a significant reduction of heat transfer. Simulations based on CFD or lumped parameter (LP) approaches use correlations to estimate heat and mass transfer due to condensation. In this work, we are interested in theoretical correlations based on the diffusion layer theory using heat and mass transfer analogy. Among the variables and parameters affecting wall condensation, the effect of the gas mixture properties in particular the diffusion coefficient modeling is investigated. Test cases of steam injection into an enclosure filled with air or air-hydrogen mixture are simulated with a LP code using two different correlations for the evaluation of heat and mass transfer. Each correlation is based on different formulations of the effective diffusion coefficient. The ISP47 test performed in the MISTRA facility was used for validation purpose. The results showed that the addressed heat and mass transfer correlations underestimate the condensation rate in the steady state. Furthermore, the impact of the effective diffusion coefficient modeling on the heat and mass transfer turned out to be significant compared to the effect of the heat and mass transfer correlation.

Numerical investigation for the optimal placements of rectangular vortex generators for improved thermal performance of fin-and-tube heat exchangers

In the present work, a numerical investigation has been performed to find out the best emplacement of rectangular winglet pairs (RWPs) for the improvement of the heat transfer and flow characteristics of a fin and tube compact heat exchanger at low Reynolds number ranges from 500 to 900. The RWPs are placed at two different locations i.e., adjacent to the tubes and behind the tubes namely, common-flow-up (CFU) and common-flow-down (CFD) orientations respectively. The effect of attack angle, span angle and location of RWPs on air-side thermal hydraulic performance is investigated numerically. Area goodness factor and heat transfer rate per unit fan power are considered as the performance evaluation criteria. It is found that with the increase in attack angle and span angle heat transfer rate increases causing an increase in pressure drop penalty as well. Vortex generators behind the tube can reduce the pressure drop but with a significant reduction in heat transfer. Furthermore, MOORA (Multi-objective optimization by ratio analysis) method is employed to obtain the most expedient position and orientation of RWPs.

Experimental study of the effect of drag reducing agent on heat transfer and pressure drop characteristics

ABSTRACTThe effect of the addition of polymeric drag reducing agent on water flow heat transfer and pressure drop was studied experimentally. Polyacrylamide (PAM) aqueous solutions at different concentrations (up to 100 ppm) at fully developed turbulent flow were used in a double tube heat exchanger. In contrast to previous studies, drag reduction due to using PAM was negligible. The addition of PAM, even at low concentrations, caused significant reduction in heat transfer. Results showed that increasing PAM concentration lowered the amount of heat transfer. Maximum heat transfer reduction of 25% occurred at the concentration of 100 ppm for PAM aqueous solution.

Improvement of CUPID code for simulating filmwise steam condensation in the presence of noncondensable gases

Abstract In a nuclear reactor containment, wall condensation forms with noncondensable gases and their accumulation near the condensate film leads to a significant reduction in heat transfer. In the framework of nuclear reactor safety, the film condensation in the presence of noncondensable gases is of high relevance with regards to safety concerns as it is closely associated with peak pressure predictions for containment integrity and the performance of components installed for containment cooling in accident conditions. In the present study, CUPID code, which has been developed by KAERI for the analysis of transient two-phase flows in nuclear reactor components, is improved for simulating film condensation in the presence of noncondensable gases. In order to evaluate the condensate heat transfer accurately in a large system using the two-fluid model, a mass diffusion model, a liquid film model, and a wall film condensation model were implemented into CUPID. For the condensation simulation, a wall function approach with a heat/mass transfer analogy was applied in order to save computational time without considerable refinement for the boundary layer. This paper presents the implemented wall film condensation model, and then introduces the simulation result using the improved CUPID for a conceptual condensation problem in a large system.

Mechanical behavior and springback of acetylated particleboard made in different press times

In this study, the effects of acetylation and press time on the heat transfer from the surface to the core on the particleboard mat, springback, internal bonding, modulus of elasticity, and modulus of rupture were determined. Particles were soaked in the acetic anhydride for 24 h and heated in an oven at 120°C for 40 minutes, 3 h and 6 h to achieve three weight gain levels of 8%, 12%, and 17%, respectively. Acetylated and control boards were produced with 10% melamine urea formaldehyde (regarding the particles dry weight) and pressed for 5, 6, and 7 minutes. During the pressing process, the heat transfer to the core layer of the mat was measured by thermocouple made of Cr-Ni wires. It was found that the increase of acetylation level leads to significant reduction in heat transfer during manufacture process and develops springback which in turn, results in density loss. Findings indicated that these defects were correlated with the press time and compensated by the press time prolongation. Strength loss of the acetylated boards was due to debonding of the constituent that compensated somewhat with increasing press time.

Effect of non-condensable gas on condensation in a highly-rotating drum with a scraper

The effect of non-condensable gas on condensation in a highly rotating drum with a scraper is explored analytically. The formulation is first carried out generally, and then the application of the general formulation to the steam-air system gives heat transfer results for a wide range of operating conditions. It is found that a very small amount of non-condensable gas can induce a significant reduction in heat transfer. The reduction is accentuated at larger temperature differences, thinner condensate thickness and, to one's surprise, at higher operating pressures. By comparison, it is indicated that the present situation is more sensitive to non-condensable gas than forced convection condensation along a horizontal plate.

A study of laminar hypersonic cavity flows

An experimental investigation of the distributions of pressure, recovery factor, and heattraiisfer coefficient on 20° cones incorporating annular cavities was carried out in helium at MOO = 11 • The effects of Reynolds number, cavity length-depth ratio, and reattachment region geometry were studied. Attention was confined to the laminar, open cavity flow regime. The results showed that the pressure was virtually constant on the cavity floor for a cavity length-depth ratio less than seven. A definite pressure gradient appeared at higher length-depth ratios. The recovery factor was virtually constant within the cavity and downstream and was very close to the laminar attached-flow value. The distribution of heattransfer coefficient showed a pronounced minimum on the cavity floor, falling to about 10 to 20% of the attached-flow value. The heat-transfer coefficient passed through a maximum in the reattachment region, then went asymptotically to a value lower than the attached-flow value. The integrated heat-transfer rate in the separated flow region was in excellent agreement with Chapman's laminar mixing layer theory, being about 55% of the attached-flow value. However, this significant reduction in heat transfer was almost nullified by the increased heat-transfer rates close to reattachment.