Enhanced Heat Transport Research Articles

Heat/mass transfer from a drop translating in steady and time-periodic electric fields: External problem

Enhancement of heat or mass transport from a spherical drop of a dielectric fluid translating in another dielectric fluid in the presence of steady and time periodic electric fields (both uniform and non-uniform) is investigated in this paper. The external problem or the limit of the majority of the transport resistance being in the continuous phase is considered. Using a finite volume formulation, the transient energy (species) conservation equation is solved for Peclet numbers (Pe) varying from 10 to 1000 and dimensionless electric field frequency (ω∗) from 10 to 50,000 using a fully implicit method. To map the infinite domain in the radial direction, an exponential transformation is employed that provides a fine grid spacing near the drop surface where sharp variations are expected and a coarser grid in the far field where low gradients prevail. The transient temperature distribution and the local and the average Nusselt numbers are obtained and the heat/mass transfer enhancement due to the application of electric field is determined. The effect of electric field is expressed in terms of L, the ratio of the maximum electric-field-induced surface velocity to the translation-induced surface velocity. For the steady and time-periodic electric fields, the heat transfer enhancement increases monotonically with Pe and with L. Also, the heat transport rate is higher when the continuous phase is more viscous compared to the dispersed phase. The steady uniform electric field gives the highest average Nusselt number for heat transfer from the drop surface to the continuous phase, followed by the non-uniform time periodic electric field and then the time periodic uniform electric field. The enhancement relative to pure translation exhibits a non-monotonic dependence on electric field frequency but the highest value is obtained at ω∗=0. Earlier studies have shown that there is significant enhancement in the heat/mass transfer in the drop interior with a time periodic electric field compared to a steady uniform electric field when the majority of the transport resistance is in the drop. The results presented here show that an opposite behavior is obtained in the drop exterior, i.e. a steady electric field provides higher heat/mass transfer enhancement compared to a time periodic electric field, when the bulk of the transport resistance is in the continuous phase. Therefore whether the steady or the time periodic electric field provides the most enhancement of heat/mass transfer for a conjugate problem will depend on the relative transport resistance in the two phases.

Plumes Dynamics and Heat Transfer over Horizontal Grooved Surfaces

This study investigates the free convection and plumes dynamics over horizontal surfaces with parallel V-grooves. The convection is studied in a tank of water with the bottom surface being a smooth or grooved surface and the top of the water surface exposed to ambient. Two groove heights were used—10 mm and 3 mm—and the experiment was done with two values of aspect ratio—2.9 and 1.8 (aspect ratio is the width of the fluid layer/height of fluid layer). Heat flux at the bottom surface was from electrical heating. Beyond a certain critical temperature difference, enhanced heat transfer is obtained on the grooved surface compared to a smooth surface. Nusselt numbers are evaluated for both smooth and grooved surfaces and correlated using modified Rayleigh numbers. Visualization shows that the enhanced heat transport in the rough cavities cannot be ascribed to the increase in the contact area; rather, it must be the local dynamics of the thermal boundary layer.

Beating synthetic cilia enhance heat transport in microfluidic channels

Using computational modeling, we probed the utility of actuated synthetic cilia for enhancing heat transport in microfluidic channels. Cilia are elastic filaments that are attached to the bottom channel wall with a constant tilt and actuated by a periodical vertical force applied to their free ends. We show that periodical oscillations of elastic cilia mix the heated fluid and create secondary flows in the microchannel that facilitate heat transport between channel walls. The magnitude of the secondary flows and the cilium deformation pattern are controlled by the frequency and amplitude of the periodic driving force. Thus, by varying the force parameters one can effectively regulate the local heat transport in ciliated microchannels.

Rough horizontal plates: heat transfer and hysteresis

To investigate the influence of a rough-wall boundary layer on turbulent heat transport, an experiment of high-Rayleigh convection in water is carried out in a Rayleigh-Benard cell with a rough lower plate and a smooth upper plate. A transition in the heat transport is observed when the thermal boundary layer thickness becomes comparable to or smaller than the roughness height. Besides, at larger Rayleigh numbers than the threshold value, heat transport is found to be increased up to 60 %. This enhancement cannot be explained simply by an increase in the contact area of the rough surface since the contact area is increased only by a factor of 40 %. Finally, a simple model is proposed to explain the enhanced heat transport.

Bifurcations in turbulent rotating Rayleigh-Bénard convection: A finite-size effect

In turbulent rotating Rayleigh-Bénard convection Ekman vortices extract hot or cold fluid from thermal boundary layers near the bottom or top plate and enhance the Nusselt number. It is known from experiments and direct numerical simulation on cylindrical samples with aspect ratio Γ ≡ D/L (D is the diameter and L the height) that the enhancement occurs only above a bifurcation point at a critical inverse Rossby number 1/Roc, with 1/Roc ∞ 1/Γ. We present a Ginzburg-Landau like model that explains the existence of a bifurcation at finite 1/Roc as a finite-size effect. The model yields the proportionality between 1/Roc and 1/Γ and is consistent with several other measured or computed system properties. Here it is used to estimate the suppression of the heat-transport enhancement by the sidewall.

Phase Field Approach to Multiphase Flow Modeling

We review the phase field (otherwise called diffuse interface) model for fluid flows, where all quantities, such as density and composition, are assumed to vary continuously in space. This approach is the natural extension of van der Waals’ theory of critical phenomena both for one-component, two-phase fluids and for partially miscible liquid mixtures. The equations of motion are derived, assuming a simple expression for the pairwise interaction potential. In particular, we see that a non-equilibrium, reversible body force appears in the Navier-Stokes equation, that is proportional to the gradient of the generalized chemical potential. This, so called Korteweg, force is responsible for the convection that is observed in otherwise quiescent systems during phase change. In addition, in binary mixtures, the diffusive flux is modeled using a Cahn-Hilliard constitutive law with a composition-dependent diffusivity, showing that it reduces to Fick’s law in the dilute limit case. Finally, the results of several numerical simulations are described, modeling, in particular, a) mixing, b) spinodal decomposition, c) nucleation, d) enhanced heat transport, e) liquid-vapor phase separation.

The nature of MIS 3 stadial–interstadial transitions in Europe: New insights from model–data comparisons

15 abrupt warming transitions perturbed glacial climate in Greenland during Marine Isotope Stage 3 (MIS 3, 60–27 ka BP). One hypothesis states that the 8–16 °C warming between Greenland Stadials (GS) and Interstadials (GI) was caused by enhanced heat transport to the North Atlantic region after a resumption of the Atlantic Meridional Overturning Circulation (AMOC) from a weak or shutdown stadial mode. This hypothesis also predicts warming over Europe, a prediction poorly constrained by data due to the paucity of well-dated quantitative temperature records. We therefore use a new evidence from biotic proxies and a climate model simulation to study the characteristics of a GS–GI transition in continental Europe and the link to enhanced AMOC strength. We compare reconstructed climatic and vegetation changes between a stadial and subsequent interstadial – correlated to GS15 and GI14 (∼55 ka BP) – with a simulated AMOC resumption using a three-dimensional earth system model setup with early-MIS 3 boundary conditions. Over western Europe (12°W–15°E), we simulate twice the annual precipitation, a 17 °C warmer coldest month, a 8 °C warmer warmest month, 1300 °C-day more growing degree days with baseline 5 °C (GDD5) and potential vegetation allowing tree cover after the transition. However, the combined effect of frequent killing frosts, <20 mm summer precipitation and too few GDD5 after the transition suggest a northern tree limit lying at ∼50°N during GI14. With these 3 climatic limiting factors we provide a possible explanation for the absence of forests north of 48°N during MIS 3 interstadials with mild summers. Finally, apart from a large model bias in warmest month surface air temperatures, our simulation is in reasonable agreement with reconstructed climatic and vegetation changes in Europe, thus further supporting the hypothesis.

Integrated simulation of ELM triggered by a pellet through energy absorption and transport enhancement

Two integrated core/scrape-off layer/divertor transport codes TOPICS-IB and JINTRAC with links to MHD stability codes are coupled with models of pellet injection to clarify effects of the pellet on the behaviour of edge-localized modes (ELMs). Both codes predict the following two triggering mechanisms. Energy absorption by the pellet and its further displacement due to the E × B drift, as well as transport enhancement by the pellet, are found to be able to trigger the ELM. The ablated cloud of pellet absorbs the background plasma energy and causes a radial redistribution of pressure due to the subsequent E × B drift. Further, the sharp increase in local density and temperature gradients in the vicinity of ablated cloud causes the transient enhancement of heat and particle transport. Both mechanisms produce a region of increased pressure gradient in the background plasma profile within the pedestal, which triggers the ELM. The mechanisms have the potential to explain a wide range of experimental observations.

New upper bounds and reduced dynamical modeling for Rayleigh–Bénard convection in a fluid saturated porous layer

New rigorous upper bounds on the enhancement of heat transport in a fundamental model of porous medium convection are presented. The computational results are applied to the development of finite dimensional dynamical systems models of porous medium convection. The number of degrees of freedom in the full dynamics and the dimension of potentially faithful reduced descriptions are discussed.

Aerothermodynamics of a High-Pressure Turbine Blade With Very High Loading and Vortex Generators

Abstract AITEB-2 is a project where aerothermal challenges of modern high pressure turbine designs are analyzed. One of the scopes of the project is to allow for new gas turbine designs with less parts and lighter jet engines by increasing the blade pitch and therefore the aerodynamic blade loading. For transonic profiles, this leads to very high velocities on the suction side and shock induced separation is likely to occur. The total pressure loss increase due to flow separation and strong shocks, as well as the underturning of the flow, limits the increase of the blade pitch. In this paper, experiments using a linear turbine blade cascade with high aerodynamic loading are presented. The blade pitch is increased such that at design conditions, a strong separation occurs on the suction side. The experiments were run at high subsonic exit Mach numbers and at Reynolds numbers of 390,000 and 800,000. In order to reduce the flow separation and the aerodynamic losses, air jet vortex generators are used, which create streamwise vortices prior to the separation start. Since in high pressure turbine blades film cooling is widely used, also the influence of film cooling both with and without using vortex generators is analyzed. Film cooling is provided on the suction side by two rows of cylindrical holes. This paper provides an analysis of the influence of different main flow conditions, film cooling, and vortex generators on total pressure loss, heat transfer and film cooling effectiveness. The experiments show that the vortex generators, as well as the film cooling reduce flow separation and total pressure losses. The effects are also seen in the local heat transfer, especially with enhanced heat transport in the region with flow separation. The cases presented in this paper deal with complex flow phenomena, which are challenging to be predicted with modern numerical tools correctly. Therefore, the experimental data serve as a comprehensive database for validation of simulation tools in the AITEB-2 project.

Effects of polymer additives on Rayleigh-Taylor turbulence

The role of polymer additives on the turbulent convective flow of a Rayleigh-Taylor system is investigated by means of direct numerical simulations of Oldroyd-B viscoelastic model. The dynamics of polymer elongations follows adiabatically the self-similar evolution of the turbulent mixing layer and shows the appearance of a strong feedback on the flow which originates a cutoff for polymer elongations. The viscoelastic effects on the mixing properties of the flow are twofold. Mixing is appreciably enhanced at large scales (the mixing layer growth rate is larger than that of the purely Newtonian case) and depleted at small scales (thermal plumes are more coherent with respect to the Newtonian case). The observed speed up of the thermal plumes, together with an increase of the correlations between temperature field and vertical velocity, contributes to a significant enhancement of heat transport. Our findings are consistent with a scenario of drag reduction induced by polymers. A weakly nonlinear model proposed by Fermi for the growth of the mixing layer is reported in the Appendix.

Carbon nanotube forest growth on NiTi shape memory alloy thin films for thermal actuation

Actuation frequencies in thermally triggered Shape Memory Alloy (SMA) thin films are limited by the slow heat transport into/out of the films. Carbon Nanotubes (CNTs) are known to exhibit an exceptionally high thermal conductivity. Thus, we propose to thermally contact SMA films with CNTs to increase SMA actuation frequencies by enhanced heat transport through the CNTs. The basic requirement for this envisaged nanotube application is to obtain CNT forest growth on a SMA material while retaining a reversible martensitic transformation, as required for Shape Memory Effect exploitation. We show how such growth can be achieved on thin films of the SMA material NiTi. Future work is needed to measure thermal properties and obtainable cycling frequencies of CNT-SMA structures.

Revolutionizing heat transport enhancement with liquid metals: Proposal of a new industry of water-free heat exchangers

Water is perhaps the most widely adopted working fluid in conventional industrial heat transport engineering. However, it may no longer be the best option today due to the increasing scarcity of water resources. Furthermore, the wide variations in water supply throughout the year and across different geographic regions also makes it harder to easily access. To address this issue, finding new alternatives to replace water-based technologies is imperative. In this paper, the concept of a water-free heat exchanger is proposed and comprehensively analyzed for the first time. The liquid metal with a low melting point is identified as an ideal fluid that can flexibly be used within a wide range of working temperatures. Some liquid metals and their alloys, which have previously received little attention in thermal management areas, are evaluated. With superior thermal conductivity, electromagnetic field drivability, and extremely low power consumption, liquid metal coolants promise many opportunities for revolutionizing modern heat transport processes: serving as heat transport fluid in industries, administrating thermal management in power and energy systems, and innovating enhanced cooling in electronic or optical devices. Furthermore, comparative analyses are conducted to understand the technical barriers encountered by advanced water-based heat transfer strategies and clarify this new frontier in heat-transport study. In addition, the unique merits of liquid metals that could lead to innovative heat exchanger technologies are evaluated comprehensively. A few promising industrial situations, such as heat recovery, chip cooling, thermoelectricity generation, and military applications, where liquid metals could play irreplaceable roles, were outlined. The technical challenges and scientific issues thus raised are summarized. With their evident ability to meet various critical requirements in modern advanced energy and power industries, liquid metal-enabled technologies are expected to usher a new and global era of water-free heat exchangers.

Effects of geometric design on thermal performance of star-groove micro-heat pipes

This work is aimed to study the effects of geometric design on the thermal performance of star-groove micro-heat pipes. A mathematical one-dimensional, steady-state model is developed from the first principles where the continuity, momentum, and energy equations of the liquid and vapor phases, together with the Young–Laplace equation, are solved numerically to yield the heat and fluid flow characteristics of the micro-heat pipe. The heat transport capacity and the corresponding optimal charge level of the working fluid are evaluated for different geometric designs and operating conditions. The unique geometrical design of the star-groove micro-heat pipes provides a better insight into the effects of various geometrical parameters, such as the cross-sectional shape, the acuteness of the corner apex angle, the number of corners, cross-sectional area and total length. Comparisons of the performance between the star-groove and regular polygonal micro-heat pipes are performed and the factors contributing to the enhancement of heat transport due to the variations of geometrical parameters are identified and discussed.

Pliocene climate change of the Southwest Pacific and the impact of ocean gateways

The transition from the early Pliocene “Warmhouse” towards the present “Icehouse” climate and the role of Gateway dynamics are intensively debated. Both, the constrictions of the Central American Seaway and the Indonesian Gateway affected ocean circulation and climate during the Pliocene epoch. Here, we use combined δ18O and Mg/Ca ratios of planktonic foraminifera (marine protozoa) from surface and subsurface levels to reconstruct the thermal structure and changes in salinities from the Southwest Pacific Deep Sea Drilling Project (DSDP) Site 590B from 6.5 to 2.5Ma. Our data suggest a gradual cooling of ~2°C and freshening of the sea surface during ~4.6–4Ma with an increased meridional temperature gradient between the West Pacific Warm Pool and the Southwest Pacific when the closing of the Central American Seaway reached a critical threshold. After ~3.5Ma, the restricted Indonesian Gateway might have amplified the East Australian Current, allowing enhanced heat transport towards the Southwest Pacific with reduced meridional temperature gradients when the global climate gradually cooled. At the same time our data suggest a cooling and freshening of Subantarctic Mode Water (SAMW) or/and an increased northward flow of SAMW towards Site 590B, possibly a first step towards the present Antarctic Frontal System.

Does confined turbulent convection ever attain the ‘asymptotic scaling’ with 1/2 power?

We examine turbulent thermal convection at very high Rayleigh numbers using helium gas in a cylindrical container of diameter-to-height aspect ratios of 1 and 4, and confirm that the Nusselt number, Nu, follows an approximately 1/3 power of the Rayleigh number, Ra, up to some value of Ra that depends on various experimental conditions, such as the nearness to the critical point of helium as well as the aspect ratio. An enhancement of heat transport occurs for higher Ra, coinciding with a substantial increase in Prandtl number as well as in various measures of Boussinesq conditions, and marking the transition from the region in which Nu≈0.064Ra1/3 to another in which Nu≈0.078Ra1/3. By necessity, the Nusselt number in the transition region increases more steeply than 1/3. The transitional slope, which happens to be close to 1/2, does not occur at unique values of Ra for given Pr and so should not be mistaken for the ‘ultimate regime’ of Kraichnan. By comparing various experiments performed under different conditions (but in the apparatus with the same horizontal surfaces), we empirically find that substantially negative values of a non-dimensional parameter related to fluid conductivity and viscosity correlate well with the observed enhancement of heat transport.

Three-dimensional effects in counterflow heat-recirculating combustors

Three-dimensional (3D) numerical simulations of spiral counterflow Swiss roll heat-recirculating combustors were performed including gas-phase conduction, convection and chemical reaction of propane–air mixtures, solid-phase conduction and surface-to-surface radiation. These simulations showed that in 3D, results are surprisingly similar with or without a turbulence model activated because without turbulence, Dean vortices form in the curved channels which enhance heat transport and thus heat recirculation by nearly the same amount as turbulence does. Turbulence enhances the apparent viscosity to the point that, when the turbulence model is activated, the Dean vortices are not formed at the moderate Reynolds numbers studied in this work. Predictions of both 3D models are in good agreement with experiments with respect to extinction limits and temperature distributions. Comparing 3D to two-dimensional (2D) simulations employing a simple model for the out-of-plane heat losses, the turbulence model is found to be essential for accurate predictions in 2D because Dean vortices cannot form in 2D simulations. Predictions obtained using a 1-step chemical reaction model with the pre-exponential term adjusted to obtain agreement between model and experiments at one test condition are compared to a 4-step reaction model developed for flow reactors. The latter is found to provide good agreement with experiments with no adjustable parameters, which is argued to be plausible because the conditions inside heat-recirculating combustors are closer to those of flow reactors than propagating flames.

Analysis of heat recovery and heat transfer within entrapped porous triangular cavities via heatline approach

A finite element analysis is carried out to investigate the heat transfer details in entrapped porous triangular cavities which are formed between a pair of square tubes for a situation where hot fluid passes through the stack and cold fluid is entrapped within the triangular cavities. The present numerical procedure is performed over a wide range of parameters of Darcy number, Da (10 −5–10 −1), Prandtl number, Pr (0.015–1000) and Rayleigh number, Ra (10 3–5 × 10 5). At low Darcy number ( Da = 10 −5), heat transfer is primarily due to conduction for both the triangles since the heatlines are normal to the isotherms. But, as Darcy number increases conduction dominant mode changes to convection dominant. At lower Prandtl number ( Pr = 0.015) with Da = 10 −3, circulations are formed within the upper triangular cavity. Multiple cells of heatlines with high intensity are also observed due to the multiple circulation cells in fluid flow and isotherms show oscillatory pattern within the upper triangular cavity. Dense heatlines are also found near the bottom portion of the inclined walls at higher Da (10 −3–10 −1) for the upper triangle, indicating enhanced heat transport. It is interesting to observe that change of Prandtl number does not show much variation in heating patterns within the lower triangle. Heat transfer rates are obtained in terms of local ( Nu l , Nu h ) and average Nusselt numbers ( Nu l ¯ , Nu h ¯ ) . It is observed that, at low Prandtl number with high Darcy number, local Nusselt numbers of the upper triangle follow oscillatory pattern due to the multiple circulations whereas local Nusselt numbers follow monotonic trend for the lower triangle irrespective of Prandtl number and Darcy number. Average Nusselt number shows overall larger heat transfer rate for upper triangular cavity comparing to the lower triangular cavity at Da = 10 −3. The decreasing trend of average Nusselt number for lower and upper triangles at higher Ra is due to smaller local Nusselt number at specific spatial positions.

Polymer Heat Transport Enhancement in Thermal Convection: The Case of Rayleigh-Taylor Turbulence

We study the effects of polymer additives on turbulence generated by the ubiquitous Rayleigh-Taylor instability. Numerical simulations of complete viscoelastic models provide clear evidence that the heat transport is enhanced up to 50% with respect to the Newtonian case. This phenomenon is accompanied by a speed-up of the mixing layer growth. We give a phenomenological interpretation of these results based on small-scale turbulent reduction induced by polymers.

Effective Thermal Properties of Multilayered Systems with Interface Thermal Resistance in a Hyperbolic Heat Transfer Model

One-dimensional thermal wave transport in multilayered systems with an interface thermal resistance is studied under the framework of the Cattaneo–Vernotte hyperbolic heat conduction model, considering modulated heat excitation under Dirichlet and Neumann boundary conditions. For a single semi-infinite layer, analytical formulas useful in the measurement of its thermal relaxation time as well as additional thermal properties are presented. For a composite-layered system, in the thermally thin regime, with the Dirichlet boundary condition, the well known effective thermal resistance formula is obtained, while for the Neumann problem, only the heat capacity identity is found. In contrast, in the thermally thick case, an analytical expression for both Dirichlet and Neumann conditions is obtained for the effective thermal diffusivity of the whole system in terms of the thermal properties of the individual layers and their interface thermal resistance. The limits of applicability of this equation, in the thermally thick regime, are shown to provide useful and simple results in the characterization of layered systems and that they can be reduced to the results obtained using the Fourier approach. The role of the thermal relaxation time, the interface thermal resistance, and the implications of these results in the possibility of enhancement in heat transport are discussed.