Development Of Thermal Boundary Layer Research Articles

Marginal stability analyses for thermochemical convection and its implications for the dynamics of continental lithosphere and core-mantle boundary regions

Summary Significant compositional differences may exist in the lithospheric mantle and above the core-mantle boundary (CMB) relative to the ambient mantle. The intrinsic density differences may affect the development of thermal boundary layer (TBL) instabilities associated with lithospheric delamination and formation of thermochemical plumes. In this study, we explored the instability of two-layer thermochemical fluid using two different techniques: marginal stability analysis with a propagator-matrix method and finite element modeling. We investigated both the instabilities in lithospheric mantle (i.e., lithospheric instability) and the mantle above the CMB (i.e., plume-forming instability) using a background temperature Tbg(z) with the TBL. For lithospheric instability, we found that two-layer fluid with free-slip boundary conditions mainly undergoes the same three different convective modes (i.e., two oscillatory convection modes and one layered convection regime) as that with no-slip boundary condition reported in Jaupart et al., (2007). However, with free-slip boundary conditions, the transitions between these convection modes occur at larger values of buoyancy number B. Free-slip boundary conditions lead to smaller critical Rayleigh number Rac, but larger convective wavelength and oscillation frequency ωc, compared with those with no-slip boundary conditions. Our numerical modeling results demonstrate that Rac and ωc predicted from the classical marginal stability analyses using Tbg(z) with TBL temperature may have significant errors when the oscillatory period is comparable with or larger than the timescale of lithospheric thermal diffusion that causes Tbg(z) to vary with time significantly. In this case, using a more gently sloped background temperature profile ignoring the TBL temperature, the stability analysis predicts more accurate stability conditions, thus presenting an effective remedy to the stability analysis. For plume-forming instability, because of the reduced viscosity in the hot and compositionally dense bottom layer, the transition to the layered convection occurs at significantly smaller B values, and in the oscillatory convection regime, Rac is larger but ωc is smaller, compared with those for lithospheric instability. Finally, our study provides a successful benchmark of numerical models of thermochemical convection by comparing Rac and ωc from numerical models with those from the marginal stability analysis.

Numerical investigation of microchannel heat sink with novel ogive shape ribs

In this study, 3D numerical conjugate heat transfer modeling is used to investigate the thermal and hydraulic characteristics of the microchannel heat sink (MCHS) with different configurations of novel ogive shape ribs on channel walls. It was found that new proposed MCHS configurations with ogive ribs have a high Nusselt number as compared to the smooth MCHS because ogive ribs enhance the heat dissipation between channel walls and fluid by continuously interrupting the thermal boundary layer development. MCHS configuration with ogive ribs mounted on bottom wall improves the Nusselt number of smooth MCHS by 1.13–1.87 times, while MCHS with ogive ribs mounted on both side walls and MCHS with ogive ribs mounted on bottom channel wall improve Nusselt number by 1.12–1.70 and 1.08–1.59 times, respectively, at Re 100–1000. In terms of thermal enhancement factor criterion, the MCHS with ogive ribs on side walls shows superior performance at Re 100–300 and has the highest thermal enhancement factor. While MCHS with ogive ribs on bottom wall outperformed other configurations in terms of thermal enhancement factor at Reynold number > 300. A maximum thermal enhancement factor of 1.42 is reported for MCHS with bottom wall ribs ogive at Re = 1000.

Modified hexagonal pin fins for enhanced thermal-hydraulic performance of micro-pin fin heat sinks

PurposeThis numerical study aims to investigate the modification of the hexagonal pin fin geometry to enhance both the thermal and hydraulic performance of the copper micropin fin heat sink with single-phase water coolant in a laminar regime. The heat sink performance evaluation criteria have been investigated for the parametric effects of vertex angle θ (10–120) and relative length (RL) (0.25–9) of hexagonal pin fins.Design/methodology/approachTo carry out research and reduce the computational cost, only one heat sink unit is simulated and analyzed using periodic boundary conditions on the side walls and includes a hexagonal pin fin and half channel on both sides to reflect the structural characteristics completely. The governing equations are also solved using finite volume method.FindingsThe results reveal that θ = 60 and RL = 1 yield the optimum thermal performance and heat sink performance is significantly influenced by the vertex angle and RL. The modified hexagon geometry improves fluid flow behavior by reducing the volume of the recirculation region behind the pin fin, preventing its effects on the downstream pin fins and restricting the thermal boundary layer development on its straight side. At Re = 1,000, the modified geometry enhances the average Nusselt number by 24.46% and the thermal performance factor by 23.89%, demonstrating the potential of modified hexagonal pin fins to enhance micropin fin heat sink performance.Originality/valuePrior studies suggest using the pin fins with a regular hexagonal cross-section to obtain better thermal performance. However, this comes with a higher pressure drop penalty. The modification of the hexagonal pin fin geometry has been investigated in this numerical study to enhance both the thermal and hydraulic performance of the micropin fin heat sink. Because little attention has been paid to the modification of the regular hexagonal pin fins, as a geometry inspired by natural honeycomb structures, its design optimization is relatively scarce, and a gap was felt in this field.

Simultaneous development of the hydrodynamic and thermal boundary layers of mixed convective laminar flow through a horizontal tube with a constant heat flux

The thermal and hydrodynamic features of developing mixed convective laminar flow in a long horizontal tube were numerically and experimentally investigated. The tube was heated at different constant heat fluxes and had an inner diameter of 11.52 mm and a total length of 9.5 m. Numerical simulations were conducted using ANSYS-Fluent 19.3 with a highly structured mesh and accurate temperature-dependant thermophysical properties for water. The studied parameters consisted of the wall temperatures, Nusselt numbers, vorticity, velocity and temperature gradients, hydrodynamic and thermal boundary layer development, secondary flow, as well as significant momentum terms. By analysing the vorticity characteristics, it was found that our conventional understanding of the merging boundary layer in internal tube flows had to be modified. Subsequently, methods were proposed to determine the development of the hydrodynamic and thermal boundary layers, as well as the momentum terms. The experimental and numerical results indicated that the local mixed convective Nusselt numbers decreased near the tube inlet but then increased along the tube length as secondary flow increased significantly. Furthermore, an increase in secondary flow led to a decreased thermal entrance length but increased hydrodynamic entrance length. By analysing the simultaneous development of the hydrodynamic and thermal boundary layers, it was revealed that the entrance region of a mixed convective laminar flow could be divided into six regions: (1) hydrodynamic inlet region, (2) buoyancy development region, (3) buoyancy dominating region, (4) buoyancy settling region, (5) hydrodynamically stabilising region, and (6) fully developed region.

Numerical Study of Turbulent Mixed Convection in a Square Lid Driven Cavity with an Inside Hot Bloc

In this study, we are interested in the two-dimensional numerical simulation of the turbulent mixed convection in the case of a square with two side lid-driven cavity containing a hot obstacle. The transfer equations coupled with those of the K - ε closure model and the boundary conditions were presented and discretized using the finite volume method. The coupling between the velocity and pressure fields is achieved by the SIMPLE algorithm. The technique of line-by-line scanning with the Thomas algorithm (TDMA) is used for the iterative resolution of discretized equations. The control parameters of the present study are the temperature gradient between the hot walls and the cold walls, and the speed imposed on the mobile walls. Streamlines generally show flow characterized by the presence of two counter-rotating cells. The areas adjacent to the isothermal walls and to the moving walls are the site of the development of thermal and dynamic boundary layers, where significant temperature and velocity gradients have been observed, subsequently influencing the profiles of turbulent quantities such as turbulent viscosity, the production and dissipation of turbulent kinetic energy and the intensity of turbulence.

Vibration effects on heat transfer characteristics of supercritical pressure hydrocarbon fuel in transition and turbulent states

In this paper, the heat transfer characteristics of supercritical pressure hydrocarbon fuel RP-3 in transition and turbulent flow states flowing through horizontal vibrating tubes were experimentally investigated. Effects of system pressure, heat flux, mass flow rate, inlet temperature, acceleration and frequency on heat transfer with vibration were studied. The results indicated that the vibration enhance heat transfer significantly in the entrance section, as the development of thermal boundary layer was disturbed by vibration and the transition of flowing state was promoted as well. It was also found that the heat transfer enhancement became weaker with the Reynolds number increasing and finally disappears (Re>12000). In addition, the experiments showed that the heat transfer was extremely enhanced in some certain frequencies, and it displayed similar trend of convective heat transfer for different x/d position along the tube. Finally, a new heat-transfer empirical correlation describing the vibration effects on heat transfer is proposed based on experimental results.

Effect of obliquity on impingement heating characteristics of co-axial swirling flame jet - An experimental and numerical investigation

Flame jet impingement has been studied as an indispensable technique for direct heating applications suitable for metal processing industries. The current experimental and numerical investigation pursues to unfold the fluid dynamics and heat transfer characteristics for an obliquely impinging co-axial swirling flame jet. Co-axial flame used in the present work contains outer swirling flame together with an inner non-swirling flame. Steady state heat balance technique using one dimensional energy balance across the impingement plate has been employed to determine the local heat flux distribution. Due to obliquely impinging swirling jet, development of an asymmetric thermal boundary layer takes place on the target surface. The geometry being studied here develops a complex flow field which has also been investigated numerically using RANS based ‘k-e’ RNG turbulence model. The effect of various operating parameters such as inclination (θ = 0°, 15°, 30° and 45°), impingement distance (Z/Dh = 3, 5, 7 and 9), Reynolds number (Re(s) = 2500, 4000, 5500, 7000 and 8500) and equivalence ratio (ϕ = 0.9, 1.0, 1.1 and 1.2) have been studied on the flow field development and resulting heat transfer distribution.

Sonicated direct contact membrane distillation: Influence of sonication parameters

The effect of sonication on the performance of the direct contact membrane distillation (DCMD) is studied in this work. A sonicated DCMD module equipped with a commercial PTFE membrane is built and tested in an immersed temperature-controlled water bath. High fidelity transient and thermally conjugated CFD model is also developed for the DCMD unit accounting for the sonication. Flow metrics, including temperature polarization coefficient (TPC), heat- and mass- flux under different sonication frequencies and amplitudes were evaluated. Parametric study showed a clear improvement in the DCMD system metrics and more pronounced at higher temperature. It was observed that the application of sonication impedes the development of both kinetic and thermal boundary layers due to ultrasonic-induced mixing. The results showed that the application of sonication to DCMD at the intensity of 4525 W/m2 resulted in increase of mass flux from 4.25 L/m2.h to 8.1 L/m2.h corresponding to 90% increase. It is economically unviable due to electrical power demand, i.e., ~ 35 kW.h/m3 (~$5/m3), but can be feasible below 474 W/m2 demanding 3 kW.h/m3 (nearly $0.42/m3). It was also observed that an interesting equivalence exists between the effect of acoustic amplitude and acoustic frequency on the performance of DCMD. For example, sonication at 1 kHz with 0.5 μm amplitude produced similar results, in terms of mass flux and TPC, as sonication at 5 kHz with 0.1 μm amplitude.

Performance evaluation of high concentration photovoltaic cells cooled by microchannels heat sink with serpentine reentrant microchannels

Efficient cooling is critical to reduce cell temperatures of high concentration photovoltaic (HCPV) cells to avoid the output electrical performance degradation and lifetime reduction. In this study, we develop a novel type of microchannel heat sink (MHS) with serpentine reentrant microchannels (SRM) for efficient cooling of HCPV cells. They feature serpentine flow passages with Ω-shaped cross-sectional configurations, which contribute to promote fluid mixing and disrupt the normal development of thermal boundary layers. Thus they are able to provide excellent heat transfer characteristics and highly efficient cooling performance. By the comparison of a fin heat sink, both numerical and outdoor experimental studies were comprehensively conducted to explore the enhancement feasibility of thermal and electrical performance of HCPV cells. Results showed that the SRM reduced the cell temperatures and enhanced the temperature uniformity of HCPV cell module considerably, i.e., it presented cell temperatures of 25-31℃, much smaller than that of 45-63℃ of the fin heat sink. The temperature differences of HCPV cell modules were reduced to be less than 4.4℃. Besides, the output power increased by as high as 115%, and the electrical efficiency increased to 15–20% for the HCPV cell module with serpentine reentrant microchannels. Besides, the HCPV cell module with SRM was also found to induce smaller average cell temperatures and better electrical performance than a module with parallel reentrant microchannels (PRM). Moreover, the effects of flow rate and concentration ratio on the performance of HCPV cells with SRM were also assessed.

Enhanced thermal performance with high-amplitude intermittent impingement cooling

The advances of many future engineering applications rely on effective cooling techniques. Beyond the traditional thermal management solutions, the design potential of unsteady impingement cooling is still under-explored. As a combined experimental and numerical study, this paper reports new findings on high-amplitude intermittent impingement cooling with controlled unsteady patterns. Specifical attention was paid on the intermittent flow close time ratio. The experimental work involved unsteady cooling performance measurement with a small-scale water tunnel system. Unsteady Reynolds Averaged Navier-Stokes Simulation (URANS) was conducted to illustrate the unsteady flow physics, and to evaluate the cooling performance at a wider range of flow conditions (average Reynolds number 2800 < Rem < 10,000, pulsating frequency 0.1 Hz < f < 2 Hz, close time ratio 0.2 < γ < 0.8). Both experimental and numerical data confirm a remarkable improvement of overall cooling efficiency by high-amplitude intermittent impingement flow. Especially around the wall jet region, the enhancement can reach as high as 50%. The generation and interaction of vortex rings break the development of thermal boundary layer, and enhance the generation of near wall turbulence, especially for the wall jet region. Saving in coolant consumption with high-amplitude intermittent impingement cooling technique in practice is also demonstrated. The novel concept presented in this paper can be applied to a wide range of applications including electronic cooling, deicing, gas turbine blade cooling, etc.

Heat transfer prediction of granular flow in moving bed heat exchanger: Characteristics of heat transfer enhancement and dynamic control

To realize the heat transfer between granular flow and sCO2 cycle, moving bed heat exchangers have been widely applied in Concentrating Solar Power systems. This work developed a prediction model for heat transfer enhancements and dynamic control strategies in a shell-and-plate moving bed heat exchanger. For fast evaluation of heat transfer performances in granular flow, the continuum model was adopted with different thermal resistances. The wall effect of near-wall region was modified by contact thermal resistance, and the equivalent heat diffusion in bulk region was reflected by penetration thermal resistance. The results showed that, the decrement of penetration thermal resistance can improve heat transfer effectively. To weaken the penetration thermal resistance, the thinner thermal boundary layer and the higher effective thermal conductivity are urgently required. The staggered arrangement is a valid way to limit the development of thermal boundary layer. The bank of heat exchanger plates offset horizontally from the previous bank, and the re-evolution of temperature distributions is equal to the equivalent thermal mixing between different banks. Meanwhile, the effective thermal conductivity can be improved by particle self-diffusions perpendicular to the major flow direction. For the particle migrations, the local disturbances can be applied. The relatively decrement of penetration thermal resistance can achieve 45.7% at a disturbance intensity of 5 × 10−7 m2/s. The outlet temperature of sCO2 can be well controlled by the transient temperature–velocity feedback method with relaxation coefficients. The relatively decrement of temperature fluctuation is larger than 80% with a relaxation coefficient of 40.

Experimental investigation of thermal boundary layers and associated heat loss for transient engine-relevant processes using HRCARS and phosphor thermometry

Design of efficient, downsized piston engines requires a thorough understanding of transient near-wall heat losses. Measurements of the spatially and temporally evolving thermal boundary layer are required to facilitate this knowledge. This work takes advantage of hybrid fs/ps rotational coherent anti-Stokes Raman spectroscopy (HRCARS) to measure single-shot, wall-normal gas temperatures, which provide exclusive access to the thermal boundary layer. Phosphor thermometry is used to measure wall temperature. Measurements are performed in a fixed-volume chamber that operates with a transient pressure rise/decay to simulate engine-relevant compression/expansion events. This simplified environment is conducive for fundamental boundary layer and heat transfer studies associated with engine-relevant processes. The thermal boundary layer development and corresponding heat losses are evaluated within two engine-relevant regimes: (1) an unburned-gas regime comprised of gaseous compression and (2) a burned-gas regime, which includes high-temperature compression and expansion processes. The time-history of important boundary layer quantities such as gas / wall temperatures, boundary layer thickness, wall heat flux, and relative energy lost at the wall are evaluated through these regimes. During the mild unburned-gas compression, Tcore increases by 30 K and a thermal boundary layer is initiated with thickness δT ~ 200 μm. Wall heat fluxes remain below 6 kW/m2, but corresponds to ~6% energy loss per ms. In the burned-gas regime, Tcore resembles adiabatic flame temperatures, while Twall increases by 16 K. A thermal boundary layer rapidly develops as δT increases from 290 to 730 μm. Energy losses in excess of 25% occur after flame impingement and slowly decay to ~10% at the end of expansion. Measurements also resolve thermal mixing of fresh- and burned gases during expansion, which yield strong temperature reversals in the boundary layer. Findings are compared to canonical environments and demonstrate the transient thermal boundary nature during engine-relevant processes.

Effect of interface curvature on isothermal heat transfer in a hydrophobic microchannel with transverse ribs and cavities

Superhydrophobic microchannels have been found to deliver remarkable frictional flow resistance reduction owing to entrapped air on its surface micro texture. However, the entrapped air may adversely affect the heat transfer performance of the channel due to the increased thermal resistance offered by air. Hence the suitability of superhydrophobic microchannels in micro-scale heat transfer applications can only be ascertained through a combined thermo-hydraulic performance analysis which provides the trade-off between the decreased frictional resistance and increased thermal resistance. In this work, the thermo-hydraulic performance of a superhydrophobic microchannel consisting of transverse ribs and cavities is numerically investigated, at isothermal wall conditions, using CFD software Ansys-Fluent. The ribs and cavities that make up the channel wall can be represented by alternate no-slip (isothermal rib) and shear-free (adiabatic meniscus) zones. The liquid-gas interface formed at the cavity that separates the core flow and entrapped air forms convex, flat or concave menisci depending on the Laplace pressure. The effect of menisci shape on the combined thermo-hydraulic performance is studied for different gas-fractions and protrusion angles. The Nusselt number (Nu), Poiseuille number (fRe) and its ratio (Nu/fRe), are estimated for each case under convective heat transport settings. Increased shear-free fraction leads to an increase in the thermo-hydraulic performance. The concave menisci exhibited a uniform Nu irrespective of the protrusion angle, since the liquid near the interface is almost stagnant between successive no-slip riblets. In contrast, the convex interface demonstrated a drop in Nu with the rise in protrusion angle. The ribbed hydrophobic channel exhibited better performance than a no-slip classical channel having uniform heating for a protrusion angle range of -20°<0<+20° at different shear-free fractions. Moreover, the maximum thermo-hydraulic performance is observed for a nearly flat profile, which is in-line with the previous experimental observations. The thermal boundary layer development over the ribs and cavities is found to significantly influence the assessment of Nu and consequently the typical usage of a periodic boundary condition induces appreciable error in Nu estimation. For instance, a hydrophobic microchannel formed with 20 repeating units of ribs and cavities exhibited an error of 27% in Nu estimation in comparison with the asymptotic Nu, at a moderate Reynolds number of 100. Furthermore, two-phase VOF simulations are performed to compare the heat transfer characteristics of ribbed channels with and without the presence of entrapped air inside the cavities. Finally, the suitability of shear-free and adiabatic conditions is underscored through multi-phase simulations.

Intensified single-phase forced convective heat transfer with helical-twisted tube in coil heat exchangers

The twisting structure is proposed to enhance the thermal performance of a coil heat exchanger equipped with the helical tube. Actually, it intensifies the fluid mixing and breaks continuously increased thickness of thermal boundary layers. Firstly, experiments are conducted on a smooth helical tube, and the obtained results are validated using correlations proposed by other researchers. A reasonable agreement within ±7% is detected. Then, simulations are carried out to apprise the benefits of helical-twisted tube as compared to the original geometry, i.e. helical tube. It is found that the twisted walls prevent the development of thermal boundary layers through the flow direction and change continuously the location and strength of generated secondary flows and velocity contours, leading to more uniform temperature. At the studied range of Reynolds number (600 ≤ Re ≤ 1200), the Nusselt number enhancement of 14.2% and the friction factor augmentation of 7.7% are recorded for a model at middle levels of design parameters. Finally, a parametrization analysis is performed to find the effects of each specific design parameter (helical-diameter, helical-pitch, and twist-pitch). It is observed that the helical-diameter has the highest impacts on thermal–hydraulic characteristics of the helical-twisted tube, and the twist-pitch and helical-pitch come in the second and third, respectively. The overall hydrothermal performance improves as both the helical-diameter and the twist-pitch are decreased. The maximum performance index of 1.98 is obtained for the model with the helical-diameter of 50 mm at the Reynolds number of 900.

The staged development of a horizontal pipe flow at supercritical pressure

The axial development of heat transfer to supercritical fluids in horizontal pipe has been studied both experimentally and numerically. It has been demonstrated that the flow and heat transfer exhibit a distinct three-stage development, including two entrance effect stages and an asymptotic development stage. Stage I is characterized by a rapid heat transfer coefficient reduction and is largely caused by the thermal boundary layer development. When the inlet temperature is close to the pseudo-critical value however, flow laminarisation also plays a role. Stage II is a special entrance effect, which has not been observed in flow and heat transfer at subcritical pressure condition. This is caused by the strong buoyancy effect, leading to stratification and secondary flow. Most interestingly the heat transfer of supercritical fluids in a horizontal flow can reach a pseudo-developed state stage III, where the flow and heat transfer is free from the influence of the entrance condition. This contrasts the common belief that the heat transfer of supercritical fluids can never reach an axially asymptotic condition because of the significant property variations. The existence of such a condition can be of great value for fluid-to-fluid similarity analyses.

Simultaneous 1D hybrid fs/ps rotational CARS, phosphor thermometry, and CH* imaging to study transient near-wall heat transfer processes

Near-wall transient heat transfer and flame–wall interaction (FWI) are topics of great importance in the development of downsized internal combustion (IC) engines and gas turbine technology. In this work we perform measurements using 1D hybrid fs/ps rotational CARS (HRCARS), thermographic phosphors (TGP) and CH* imaging in an optically-accessible chamber designed to study transient near-wall heat transfer processes relevant to IC engine operation. HRCARS provides single-shot gas-phase temperatures (40 µm spatial resolution and up to 3 mm wall-normal distances), while thermographic phosphors measures wall temperature and CH* measures the flame front position. These simultaneous measurements are used to resolve thermal boundary layer (TBL) development and associated gaseous heat loss for three important processes of gas–wall interactions: (1) an unburned-gas polytropic compression process, (2) FWI, and (3) post-flame and gas expansion processes. During a mild polytropic compression process, measurements emphasize that even a relatively small wall heat flux (≤5 kW/m2) yields an appreciable temperature stratification through a developing TBL. During FWI, thermal gradients induced by the flame are resolved within the TBL. Gases closest to the wall (y<0.2 mm) continue to experience thermal loading from polytropic compression until the flame is within ∼1.4 mm from the wall. Immediately afterwards, the wall first senses the flame as the wall temperature begins to increase. During FWI, gas temperatures up to 1150 K impinge on the wall, producing peak wall heat fluxes (620 kW/m2) and the wall temperature increases (ΔTwall=14 K). Gaseous heat loss in the post-flame gas occurs rapidly at the wall, yielding a TBL of colder gases extending from the wall as wall heat flux slowly decreases. HRCARS further captures the rapid cooling of gases in the TBL and core-gas during the mild expansion and exhaust process.

Similarity Solutions for Local Thermal Non-equilibrium Boundary-Layer Flows in a Fluid-Saturated Porous Medium

A general boundary-layer coordinate transformation was proposed in order to find the necessary conditions under which similarity solutions exist for free, forced and mixed convective Darcian flows subject to local thermal non-equilibrium. The aim of this study is to present the effects of local thermal non-equilibrium on the boundary-layer heat transfer in porous media, identifying each of the parameters controlling the thermal boundary-layer development. The findings of this systematic study are quite beneficial to those dealing with thermal applications using porous media. Similarity solutions are found to exist for such local thermal non-equilibrium cases as forced convection around the stagnation regions of horizontal cylinder and sphere heated according to a power function of the distance from the stagnation point, free convection over a vertical plate with its temperature increasing linearly, and free and mixed convection around the stagnation regions of isothermally heated horizontal cylinder and sphere. The study clearly indicates that the local thermal equilibrium assumption results in substantial overestimation of heat transfer rates. The local non-equilibrium effects must be considered when the modified interstitial Stanton number is less than its criterion value around 10.

Flow and Thermal Transport Through Unit Cell Topologies of Cubic and Octahedron Families

Advancements in Additive Manufacturing (AM) routes have opened several opportunities for manufacturing complex porous structures which could not have been manufactured by the conventional foaming process that essentially results in unit cells represented as Tetrakaidecahedron (TKD) shape. The regular open-cell lattices belonging to the cubic and octahedron family of structures provide wide spectrum of porosity, pore-density, permeability and surface area-to-volume ratios. These complex cell topologies can potentially be manufactured additively, making the realization of above-mentioned properties possible. For efficient heat exchanger designs, the reticulated structures made from these unit cells are expected to have higher permeability, porosity, effective thermal conductivity and interstitial heat transfer coefficient. A detailed understanding of local flow and heat transport characteristics is necessary to design unit cells with such properties. To this end, four different metal foam unit cell topologies have been investigated in the present study, viz. Tetrakaidecahedron (TKD), Cube, FD-Cube and Octet. The Cube and FD-Cube unit cell topologies investigated in the present study belong to the cubic family, where the fibers are present only along the edges and face of the unit cell volume. The TKD and Octet configurations belong to the octahedron class of lattices, where complex connectivity exists within the unit cell volume. To allow for direct comparison between these three unit cells, circular fibers are chosen, which is different from the general TKD cells (tricuspid cross-sections). The porosity was fixed 0.986 which dictated the fiber dimensions of respective unit cells, with Octet fibers being thinnest. Direct simulations have been carried out with periodic boundary conditions on unit cell faces which were parallel to the flow. Such configurations when reticulated in three dimensions are representative of porous structures which are single-cell thick in the streamwise direction. These foam shapes have been proven to be very efficient as they do not allow the thermal boundary layer development. Flow and heat transfer results obtained for above boundary conditions have also been compared with the periodic boundary condition on all the faces.The direct simulation results include pressure drop per unit length and volumetric Nusselt number (corresponding to constant heat flux and constant wall temperature boundary conditions on fibers). Flow stagnation on fibers was the main contributor for both, pressure drop increase, and interstitial heat transfer enhancement. TKD unit cell had higher flow losses and thermal performance than FD-Cube. Octet (octahedron family) configuration had the highest flow losses as well as Nusselt number compared to all other unit cell topologies, because of the large surface area offered for flow stagnation.

Investigation into convective heat transfer of nanofluids at the thermal boundary layer development region in micro-channels

The convective heat transfer at the thermal boundary layer development region in microchannels was theoretical and experimently investigated with de-ionized water and Al2O3-H2O nanofluids as the working fluids. It was found that nanofluids could effectively decreased the wall temperature and improve the heat transfer coefficient compared with the de-ionized water at the thermal boundary layer development zone. When the serious agglomeration of nanoparticles didn’t appear, increasing the concentration of Al2O3 nanoparticles could effectively increase the heat transfer enhancement effect. And the local heat transfer coefficient increase amplitude by using the nanofluids decreased along the direction of refrigerant flow. At the thermal boundary layer development zone, higher inlet velocity would get the larger increase amplitude of the heat transfer coefficient. At the thermal boundary layer development region, lower heat flux could achieve more pronounced heat transfer enhancement by nanofluids. When the thermal boundary layer developed, the heat transfer enhancement by nanofluids decreased and affected by heat flow become weak. When the mass flow rate increases, the better improvement can be achieved by nanofluids in the micro-channels. The nanoparticles have greater impact on local heat transfer coefficient in thinner thermal boundary layer, leading to the heat transfer enhancement is better at the thermal boundary layer development region in micro-channels.

Compound heat transfer enhancement of helical channel with corrugated wall structure

Newly, the use of corrugated structures in heat exchange devices has become a useful technique due to the combined merits of extended surfaces and turbulators. However, employing of this technique in curved channels is very scarce. To this aim, as a new passive compound technique, the periodical corrugated structure is proposed to enhance the overall hydrothermal performance of a helical channel with square cross-section. Firstly, calculated results for the smooth model are compared with existing empirical correlations in the literature to explore the reliability and accuracy of the current analysis. Secondly, the corrugated structure on each side (upper, lower, inner, and outer) as well as opposite sides (upper-lower and inner-outer) of the helical channel is tested individually. It is found that corrugating the side walls of the helical channel prevents the development of thermal boundary layers through the flow direction and changes continuously the location and strength of generated secondary flows and velocity contours, leading to more uniform temperature. Thirdly, three different levels of corrugation-amplitude (A = 0.8, 1.6, and 2.4 mm) and coil-diameter (D = 50, 100, and 150 mm) are considered. At the studied range of Reynolds number (100 ≤ Re ≤ 500), the best enhanced model is the case of upper-lower at A = 2.4 mm and D = 100 mm, and the maximum performance index of 1.46 is recorded for water flow through this model at Re = 400.