Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2016"

Surface Structure Enhanced Microchannel Flow Boiling

Abstract: We investigated the role of surface microstructures in two-phase microchannels on suppressing flow instabilities and enhancing heat transfer. We designed and fabricated microchannels with well-defined silicon micropillar arrays on the bottom heated microchannel wall to promote capillary flow for thin film evaporation while facilitating nucleation only from the sidewalls. Our experimental results show significantly reduced temperature and pressure drop fluctuation especially at high heat fluxes. A critical heat flux (CHF) of 969 W/cm2 was achieved with a structured surface, a 57% enhancement compared to a smooth surface. We explain the experimental trends for the CHF enhancement with a liquid wicking model. The results suggest that capillary flow can be maximized to enhance heat transfer via optimizing the microstructure geometry for the development of high performance two-phase microchannel heat sinks.

Cooling Capacity Figure of Merit for Phase Change Materials

Abstract: In this paper, a figure of merit for the cooling capacity (FOMq) of phase change materials (PCMs) is defined from the analytical solution of the two-phase Neumann–Stefan problem of melting of a semi-infinite material with a fixed temperature boundary condition (BC). This figure of merit is a function of the thermophysical properties of a PCM and is proportional to the heat transfer across the interface with the surrounding medium in this general case. Thus, it has important implications for design and optimization of PCMs for high heat-flux thermal management applications. FOMq of example low melting point metals are presented which exceed those in common nonmetallic PCMs over the same temperature range by over an order of magnitude.

Influence of Controlled Aggregation on Thermal Conductivity of Nanofluids

Abstract: Nanoparticles aggregation is considered, by the heat transfer community, as one of the main factors responsible for the observed enhancement in the thermal conductivity of nanofluids. To gain a better insight into the veracity of this claim, we experimentally investigated the influence of nanoparticles aggregation induced by changing the pH value or imposing a magnetic field on the thermal conductivity of water-based nanofluids. The results showed that the enhancement in thermal conductivity of TiO2–water nanofluid, due to pH-induced aggregation of TiO2 nanoparticles, fell within the ±10% of the mixture theory, while applying an external magnetic force on Fe3O4–water nanofluid led to thermal conductivity enhancements of up to 167%. It is believed that the observed low enhancement in thermal conductivity of TiO2–water nanofluid is because, near the isoelectric point (IEP), the nanoparticles could settle out of the suspension in the form of large aggregates making the suspension rather unstable. The magnetic field however could provide a finer control over the aggregate size and growth direction without compromising the stability of the nanofluid, and hence significantly enhancing the thermal conductivity of the nanofluid.

Entropy Generation Minimization in an Electroosmotic Flow of Non-Newtonian Fluid: Effect of Conjugate Heat Transfer

Abstract: We investigate the entropy generation characteristics of a non-Newtonian fluid in a narrow fluidic channel under electrokinetic forcing, taking the effect of conjugate heat transfer into the analysis. We use power-law model to describe the non-Newtonian fluid rheology, in an effort to capture the essential thermohydrodynamics. We solve the conjugate heat transfer problem in an analytical formalism using the thermal boundary conditions of third kind at the outer surface of the walls. We bring out the alteration in the entropy generation behavior as attributable to the rheology-driven alteration in heat transfer, coupled with nonlinear interactions between viscous dissipation and Joule heating originating from electroosmotic effects. We unveil optimum values of different parameters, including both the geometric as well as thermophysical parameters, which lead to the minimization of the entropy generation rate in the system. We believe that the inferences obtained from the present study may bear far ranging consequences in the design of various cooling and heat removal devices/systems, for potential use in microscale thermal management.

Computational Study of Saturated Flow Boiling Within a Microchannel in the Slug Flow Regime

Abstract: This paper presents a fundamental study of the flow dynamics and heat transfer induced by a slug flow under saturated flow boiling in a circular microchannel. Numerical simulations are carried out by utilizing the commercial CFD solver ANSYS FLUENT v. 14.5, with its built-in volume of fluid (VOF) method to advect the interface, which was improved here by implementing self-developed functions to model the phase change and the surface tension force. A continuous stream of bubbles is generated (by additional user-defined functions) by patching vapor bubbles at the channel upstream with a constant generation frequency. This modeling framework can capture the essential features of heat transfer in slug flows for a continuous stream of bubbles which are here investigated in detail, e.g., the mutual influence among the growing bubbles, the fluid mechanics in the liquid slug trapped between two consecutive bubbles, the effect of bubble acceleration on the thickness of the thin liquid film trapped against the channel wall and on other bubbles, and the transient growth of the heat transfer coefficient and then its periodic variation at the terminal steady-periodic regime, which is reached after the transit of a few bubble-liquid slug pairs. Furthermore, the results for a continuous stream of bubbles are found to be quite different than that of a single bubble, emphasizing the importance of modeling multiple bubbles to study this process. Finally, the outcomes of this analysis are utilized to advance a theoretical model for heat transfer in microchannel slug flow that best reproduces the present simulation data.

Pool Boiling Heat Transfer Enhancement of Water Using Brazed Copper Microporous Coatings

Abstract: A novel, high-temperature, thermally-conductive, microporous coating (HTCMC) is developed by brazing copper particles onto a copper surface. This coating is more durable than many previous microporous coatings and also effectively creates reentrant cavities by optimizing brazing conditions. A parametric study of coating thicknesses of 49–283 μm with an average particle size of ∼25 μm was conducted using the HTCMC coating to understand nucleate boiling heat transfer (NBHT) enhancement on porous surfaces. It was found that there are three porous coating regimes according to their thicknesses. The first regime is “microporous” in which both NBHT and critical heat flux (CHF) enhancements gradually grow as the coating thickness increases. The second regime is “microporous-to-porous transition” where NBHT is further enhanced at lower heat fluxes but decreases at higher heat fluxes for increasing thickness. CHF in this regime continues to increase as the coating thickness increases. The last regime is named as “porous”, and both NBHT and CHF decrease as the coating thickness increases further than that of the other two regimes. The maximum nucleate boiling heat transfer coefficient observed was ∼350,000 W/m2K at 96 μm thickness (“microporous” regime) and the maximum CHF observed was ∼2.1 MW/m2 at ∼225 μm thickness (“porous” regime).Copyright © 2015 by ASME

Computational Study of Three-Dimensional Stagnation Point Nanofluid Bioconvection Flow on a Moving Surface With Anisotropic Slip and Thermal Jump Effect

Abstract: The effects of anisotropic slip and thermal jump on the three dimensional stagnation point flow of nanofluid containing microorganisms from a moving surface have been investigated numerically. Anisotropic slip takes place on geometrically striated surfaces and superhydrophobic strips. Zero mass flux of nanoparticles at the surface is applied to achieve practically applicable results. Using appropriate similarity transformations, the transport equations are reduced to a system of nonlinear ordinary differential equations with coupled boundary conditions. Numerical solutions are reported by means of very efficient numerical method provided by the symbolic code Maple. The influences of the emerging parameters on the dimensionless velocity, temperature, nanoparticle volumetric fraction, density of motile micro-organisms profiles, as well as the local skin friction coefficient, the local Nusselt number and the local density of the motile microorganisms are displayed graphically and illustrated in detail. The computations demonstrate that the skin friction along the x-axis is enhanced with the velocity slip parameter along the y axis. The converse response is observed for the dimensionless skin friction along the y-axis. The heat transfer rate is increased with greater velocity slip effects but depressed with the thermal slip parameter. The local Nusselt number is increased with Prandtl number and decreased with the thermophoresis parameter. The local density for motile microorganisms is enhanced with velocity slip parameters and depressed with the bioconvection Lewis number, thermophoresis and Peclet number. Numerical results are validated where possible with published results and excellent correlation is achieved.

Flow and Heat Transfer in Micro Pin Fin Heat Sinks With Nano-Encapsulated Phase Change Materials

Abstract: In this paper, a 3D conjugated heat transfer model for Nano-Encapsulated Phase Change Materials (NEPCMs) cooled Micro Pin Fin Heat Sink (MPFHS) is presented. The governing equations of flow and heat transfer are solved using a finite volume method based on collocated grid and the results are validated with the available data reported in the literature. The effect of nanoparticles volume fraction (C = 0.1, 0.2, 0.3), inlet velocity (Vin = 0.015, 0.030, 0.045 m/s), and bottom wall temperature (Twall = 299.15, 303.15, 315.15, 350.15 K) are studied on Nusselt and Euler numbers as well as temperature contours in the system. The results indicate that significant heat transfer enhancement is achieved when using NEPCM slurry as an advanced coolant. The maximum Nusselt number when NEPCM slurry (C = 0.3) with Vin = 0.015, 0.030, 0.045 (m/s) is employed, are 2.27, 1.81, 1.56 times higher than the ones with base fluid, respectively. However, with increasing bottom wall temperature, the Nusselt number first increases then decreases. The former is due to higher heat transfer capability of coolant at temperatures over the melting range of PCM particles due to partial melting of nanoparticles in this range. While, the latter phenomena is due to the lower capability of NEPCM particles and consequently coolant in absorbing heat at coolant temperatures higher than the temperature correspond to fully melted NEPCM. It was observed that NEPCM slurry has a drastic effect on Euler number, and with increasing volume fraction and decreasing inlet velocity, the Euler number increases accordingly.

Hydrodynamic and Thermal Performance of Microchannels With Different In-Line Arrangements of Cylindrical Micropin Fins

Abstract: This study focuses on microheat sinks with different staggered arrangements of micro pin fins (MPFs). A rectangular microchannel with the dimensions of 5000 x 1500 x 100 mu m(3) (l' x w' x h') was considered for all the configurations while different MPF diameters, height over diameter ratio (H/D), and longitudinal and transversal pitch ratios (S-L/D and S-T/D) were considered in different arrangements. Using the ANSYS FLUENT 14.5 commercial software, the simulations were done for different Reynolds numbers between 20 and 160. A constant heat flux of 30 W/cm(2) was applied through the bottom heating section. The performances of the microheat sinks were evaluated using design parameters, namely pressure drop, friction factor, Nusselt number, and thermal-hydraulic performance index (TPI). The effect of each geometrical parameter as well as wake-pin fin interaction patterns were carefully studied using the streamline patterns and temperature profiles of each configuration. The results reveal a great dependency of trends in pressure drops and Nusselt numbers on the wake region lengths as well as the local velocity and pressure gradients. Moreover, the wake region lengths mostly contribute to the increase in obtained pressure drop and Nusselt number with Reynolds number. Although an increase in the H/D and SL/D ratios results in an increase and a decrease in pressure drop, respectively, the effect on the Nusselt number depends on other geometrical parameters and Reynolds number. A larger ST/D ratio generally results in a decrease in the pressure drop and Nusselt number. Finally, while the friction factor decreases with Reynolds number, two different trends are seen for the TPI values of configurations with the H/D ratio of 1 and 2 (D = 100 and 50 mu m). While the trend in the TPIs is increasing for Reynolds numbers between 20 and 40, it reverses for higher Reynolds numbers with a steeper slope in the configurations with the ST/D ratio of 1.5.

Mechanistic Considerations for Enhancing Flow Boiling Heat Transfer in Microchannels

Abstract: Research efforts on flow boiling in microchannels were focused on stabilizing the flow during the early part of the last decade. After achieving that goal through inlet restrictors and distributed nucleation sites, the focus has now shifted on improving its performance for high heat flux dissipation. The recent worldwide efforts described in this paper are aimed at increasing the critical heat flux (CHF) and reducing the pressure drop, with an implicit goal of dissipating 1 kW/cm2 for meeting the high-end target in electronics cooling application. The underlying mechanisms in these studies are identified and critically evaluated for their potential in meeting the high heat flux dissipation goals. Future need to simultaneously increase the CHF and the heat transfer coefficient (HTC) has been identified and hierarchical integration of nanoscale and microscale technologies is deemed necessary for developing integrated pathways toward meeting this objective.

Effect of Hydrophilic Nanostructured Cupric Oxide Surfaces on the Heat Transport Capability of a Flat-Plate Oscillating Heat Pipe

Abstract: With a surface treatment of hydrophilic cupric oxide (CuO) nanostructures on the channels inside a flat-plate oscillating heat pipe (FP-OHP), the wetting effect on the thermal performance of an FP-OHP was experimentally investigated. Three FP-OHP configurations were tested: (1) evaporator treated, (2) condenser treated, and (3) untreated. Both evaporator- and condenser-treated FP-OHPs show significantly enhanced performance. The greatest improvement was seen in the condenser-treated FP-OHP, a 60% increase in thermal performance. Neutron imaging provided insight into the fluid dynamics inside the FP-OHPs. These findings show that hydrophilic nanostructures and their placement play a key role in an OHP's performance.

Investigation on the Effect of Type and Size of Nanoparticles and Surfactant on Pool Boiling Heat Transfer of Nanofluids

Abstract: In our efforts to improve the pool boiling heat transfer of water, three sets of experiments are carried out to investigate the best coolant for heat removal among alumina, silica, and zinc oxide as nanoparticles and water as base fluid: (a) pool boiling heat transfer of γ-alumina/water nanofluid with and without surfactant in both distilled water and treated water as base fluids, (b) pool boiling heat transfer of silica/water nanofluid with two different nanoparticle sizes, and (c) pool boiling heat transfer of zinc oxide/water nanofluid with surfactant. In all the above experiments, the effect of heater surface on boiling heat transfer coefficient has been investigated by repeating the experiment using pure water on the coated surface before cleaning it. Moreover, two effective thermophysical properties of fluids, dynamic viscosity and surface tension, are measured to explain the boiling behavior of the nanofluids. The experimental results indicate that (a) the presence of γ-alumina in the base fluid enhances the pool boiling heat transfer coefficient, but sodium dodecyl sulphate (SDS) as surfactant deteriorates the performance of pool boiling heat transfer of γ-alumina/water nanofluid and (b) silica nanoparticles reduce the boiling performance of pure water. Moreover, the larger particle size of silica nanoparticles shows less reduction in heat transfer coefficient, (c) zinc oxide/water nanofluid is the best coolant among all the above-mentioned nanoparticles for heat removal.

Temperature Uniformity Improvement of an Air-Cooled High-Power Lithium-Ion Battery Using Metal and Nonmetal Foams

Abstract: In order to improve the temperature uniformity inside the battery, the effects of partially utilizing metal and nonmetal materials on the heat sink of an air-cooled Lithium-ion (Li-ion) battery module were studied. Aluminum and aluminum foam as heat conductors and ceramic, and ceramic foam as insulators were examined using two-dimensional transient numerical simulation. The effects of the length of utilizing each material to the total length of the battery pack from the inlet by assuming that the other part of the heat sink is aluminum were investigated. The results showed that using aluminum foam and ceramic as part of the heat sink decreases the temperature uniformity of the battery pack. However, using the ceramic foam at the inlet section of the heat sink improves the temperature uniformity of the battery significantly. Furthermore, partially inserting the aluminum foam inside the air flow channel from outlet was investigated, and significant enhancement on the temperature uniformity of the battery pack was found. Overall, higher temperature reduction and higher temperature uniformity were achieved inside the battery pack using the combination of both ceramic and aluminum foams. [DOI: 10.1115/1.4033811]

Geometrical Optimization and Experimental Validation of a Tripod Film Cooling Hole With Asymmetric Side Holes

Abstract: A tripod cylindrical film hole with asymmetric side holes is studied numerically and experimentally on a flat plate for higher film cooling effectiveness. First, the influences of geometrical parameters are studied and the optimum configurations of the asymmetric tripod hole are found in a design of experiments (DoE) optimization study based on an improved numerical model for film cooling prediction, in which more than 100 3D computational fluid dynamics (CFD) simulations are carried out. Then, one optimum configuration of the asymmetric tripod hole is examined experimentally using pressure-sensitive paint (PSP) measurements and compared to the experimental results of the simple cylindrical film hole and a well-designed shaped film hole. The flow and heat transferring characteristics of the asymmetric tripod holes were explored from the DoE results. The side holes can form a shear vortex system or an antikidney vortex system when proper spanwise distances between them are adopted, which laterally transports the coolant and form a favorable coolant coverage. According to the experimental results on flat plate, the optimal configuration of the asymmetric tripod hole is significantly better than cylindrical hole, especially at high blowing ratios. Furthermore, it can provide equivalent or even higher film cooling effectiveness than a well-designed shaped hole.

Impingement Heat Transfer on a Cylindrical, Concave Surface With Varying Jet Geometries

Abstract: Jet impingement is often employed within the leading edge of modern turbine airfoils to combat the extreme heat loads incurred within this region. This experimental investigation employs a transient liquid crystal technique to obtain detailed Nusselt number distributions on a concave, cylindrical surface that models the leading edge of a turbine airfoil. The effect of hole shape as well as differing hole inlet and exit conditions are investigated. Two hole shapes are studied: cylindrical and racetrack shaped holes; for each hole shape, the hydraulic diameter and mass flow rate into the array of jets is conserved. As a result, the jet’s Reynolds number (Rejet) varies between the two jet arrays. Reynolds numbers of 13600, 27200, and 40700 are investigated for the cylindrical holes and Reynolds numbers of 11500, 23000, and 34600 are investigated for the racetrack holes. Three inlet and exit conditions are investigated for each hole shape: a square edged, a partially filleted, and a fully filleted hole. The ratio of the fillet radius to hole hydraulic diameter (r / dH,Jet) is set at 0.25 and 0.667 for the partially and fully filleted holes, respectively. The relative jet–to–jet spacing (s / dH,Jet) is maintained at 8, the jet–to–target surface spacing (z / dH,Jet) is maintained at 4, the jet–to–target surface curvature (D / dH,Jet) is maintained at 5.33, and the relative jet plate thickness (t / dH,Jet) is maintained at 1.33. Results show the Nusselt number is directly related to the jet Reynolds number for both cylindrical and racetrack shaped holes. The racetrack holes are shown to provide enhanced heat transfer compared to the cylindrical holes for a set mass flow rate. The degree of filleting at the inlet and outlet of the holes affects whether the heat transfer on the leading edge model is further enhanced or degraded.Copyright © 2012 by ASME

Tapered Twisted Tape Inserts for Enhanced Heat Transfer

Abstract: Insertion of twisted tapes in smooth channels is one of the passive methods used for enhancing heat transfer. Flow and associated heat transfer characteristics of these channels are very complex. Understanding this complex flow is helpful while designing new passive methods. Numerical methods like computational fluid dynamics (CFD) are gaining much popularity for analyzing and designing these heat transfer enhancement techniques. This paper focuses on such a numerical study. The preliminary study is focused on development of numerical methodology through validation. Successive studies are aimed at development of an innovative design for twisted tape. Twisted tapes with taper angle (tapered twisted tapes, i.e., tape width decreases along the flow direction) are developed and evaluated on the basis of the performance of these tapes with those of conventional tapes. A circular tube with tapered twisted tape with a twist ratio of 3 and taper angles of 0.3, 0.4, 0.5, 0.6, and 0.7 is considered for this study along with a plain tube. Three Reynolds numbers (Re) of 8545, 11393, and 13333 are considered to examine the sensitivity of the performance. Simulations are performed with a commercially available CFD tool, ansys fluent (v14.0). Heat transfer and pressure drop results are presented in the form of Nusselt number (Nu), friction factor (f), and overall enhancement ratio (η). An increase of 17% in overall enhancement is predicted with taper angle of 0.5.

Optimization Arrangement of Two Pulsating Impingement Slot Jets for Achieving Heat Transfer Coefficient Uniformity

Abstract: In this paper, an optimization was performed to achieve uniform distribution of convective heat transfer coefficient over a target plate using two impinging slot (air) jets. The objective function is the root mean square error (Erms) of the local Nusselt distribution computed by computational fluid dynamic (CFD) simulations from desired Nusselt numbers. This pattern search minimized this objective function. Design variables are nozzle widths, jet-to-jet distance, jet-to-target plate distance, frequency of pulsations (for pulsating jets), and the flow rate. First, an inverse design is performed for two steady jets for simplicity and the obtained errors for three different desired Nusselt numbers, NuD1⁄4 7, 10, and 13, were 20.73%, 20.08%, and 22.92%, respectively. Uniform distribution of heat transfer coefficient for two steady jets was not achieved. Thus, two pulsating jets are considered. The range of design variables for pulsating state is as same as steady-state and heat transfer rates increased about 400% over steady-state due to the effects of pulsations in inlet velocity. Thus, in the pulsating state, optimization must be performed for the desired Nusselt numbers around four-times NuD in the steady-state, i.e., NuD1⁄4 28, 40, and 52. The Erms reduced less than 0.01% and distribution of heat transfer coefficient for all cases was uniform. An experimental study using an inverse heat conduction method (conjugate gradient method with adjoint equation) has been performed and the experimental results for the case of NuD1⁄4 52 are presented. The estimated distribution of Nusselt number on the target plate with the numerical distribution has around 3.2% relative error with optimal configuration. [DOI: 10.1115/1.4033616]