Contribution Of Convection Research Articles

Direct local heat flux measurement during water flow boiling in a rectangular minichannel using a MEMS heat flux sensor

Heat transfer characteristics of water flow boiling in a minichannel were investigated through the direct measurement of local heat flux using a fabricated MEMS heat flux sensor with a multi-layered structure. Local temperatures and heat fluxes in the water flow boiling were measured at a sampling frequency of 10 kHz in synchronism with the high-speed visualization of the boiling behavior. The measurement results revealed fundamental heat transfer processes, including thin liquid film evaporation, dry-out of the liquid film, transient heat conduction after dry patch rewetting, and single-phase forced convection. The thin liquid film evaporation indicated a high local heat flux that was well over 1 MW/m2 and provided a dominant contribution to the wall heat transfer. Furthermore, the post-rewetting transient heat conduction indicated a high heat flux that was higher than 1 MW/m2. However, it was insignificant in terms of the overall wall heat transfer at all the tested heat fluxes because of its short duration. The contribution of liquid-phase forced convection was also small, except at low vapor qualities.

THE EFFECT OF STEFAN FLOW ON THE MODELS OF DROPLET EVAPORATION

Droplet evaporation has been widely studied in the literature due to its key role in various applications in science and industry. The problem of droplet evaporation involves various mechanisms in both liquid and vapor phases together with the interface separating them. Modeling of this multiphase problem is not straightforward thereof studied by many researchers but in every time a few different contributing mechanisms could be highlighted. One of the pieces of this puzzle is undoubtedly the Stefan flow, which is always present during the evaporation of a liquid to an insoluble surrounding gas, yet the number of studies exploring its individual contribution to the evaporation remain very restricted. In the current study, the effect of Stefan flow is assessed by employing a recent state-of-the-art model that accounts for all pertinent physics of droplet evaporation. Results reveal that Stefan flow can be responsible for 17% of total evaporation when the droplet is placed on a high temperature substrate. Moreover, it is shown that lower performance of diffusion based models (in gas phase) can be greatly enhanced by incorporating the effect of Stefan flow into the interfacial mass flux equation. In addition, performances of existing purely diffusion and diffusion and Stefan flow based correlations in the prediction of evaporation rates are elucidated. Last but not least, under varying humidity of the surrounding gas, contribution of individual transport mechanisms in gas phase to the total evaporation rate is found to be unaffected. Based on this result, it is hypothesized that contributions of Stefan flow and natural convection have a linear dependence on the contribution of sole diffusion. The current study clearly demonstrated that Stefan flow considerably enhances the evaporation rate of droplets, especially in the case of high substrate heating. Therefore, future studies on the topic should account for the Stefan flow during the modeling of droplet evaporation.

An Experimental Investigation of Constrained Melting Heat Transfer of Nano-Enhanced Phase Change Materials in a Horizontal Cylindrical Capsule Using Thermochromic Liquid Crystal Thermography

Abstract The constrained melting of nano-enhanced phase change materials (NePCM) in a horizontal cylindrical capsule was investigated via the utilization of thermochromic liquid crystal (TLC) technique for tracking the invisible phase interfaces. A parametric study was carried out by varying both the loading of NePCM (i.e., 0 wt %, 1 wt %, and 3 wt %) and the wall superheat (at 10 °C and 30 °C), leading to a total of six cases. Numerical simulations, based on the enthalpy-porosity method, were also performed to reveal the evolutions of temperature and convective flow fields during melting. It was first shown that the numerically predicted melting front evolutions are in good agreement with the TLC imaging results. A comparison among the six cases indicated that there is a similar melting pattern that heat conduction dominates the initial stage of melting and natural convection then takes over to play a more important role when melting proceeds. With the TLC-assisted reconstruction of the melting fronts, the instantaneous melting and heat storage rates were estimated, and melting was clearly found to slow down with increasing the loading of NePCM, as a result of the dramatically increased viscosity that deteriorates the contribution of natural convection to melting heat transfer.

Hurricane Hazard Assessment Along the United States Northeastern Coast: Surface Wind and Rain Fields Under Changing Climate

Surface wind and rain fields are two significant elements of hurricane-induced hazards in coastal areas. Mitigation of losses due to hurricane wind and rain hazards has become an increasing urgent and challenging issue in light of changing climate and continued escalation of coastal population density, prompting the need for a more advanced risk analysis methodology to take global warming effects into consideration. In this study, the assessment of hurricane surface wind and rain hazards under changing climate is achieved by performing three simulation components, namely an enhanced hurricane track model to generate the synthesized storms (including a physics-based intensity model integrating sea surface temperature (SST), wind shear, and convective instability contributions), a newly developed thermal wind balance-based model to simulate the gradient wind profiles (explicitly considering environmental conditions of SST, temperature at the top of atmospheric boundary layer, and outflow temperature), and a height-resolving boundary-layer model to obtain the surface wind and rain fields (reducing inherent uncertainties associated with conventionally used gradient-to-surface wind speed conversion factors). A total of 10,000 years of hurricane events are generated for both observed (historical) and projected climate conditions, and a systematical comparison between these two scenarios is investigated. The simulation and comparison results highlight the important effects of a global warming scenario on hurricane surface wind and rain fields, and hence on critical civil infrastructure in hurricane-prone areas.

Shear Effects on Scalar Transport in Double Diffusive Convection1

Abstract In this article, we examine the effect of shear on scalar transport in double diffusive convection (DDC). DDC results from the competing action of a stably stratified, rapidly diffusing scalar (temperature) and an unstably stratified, slowly diffusing scalar (salinity), which is characterized by fingering instabilities. We investigate, for the first time, the effect of shear on the diffusive and convective contributions to the total scalar transport flux within a confined fluid layer, examining also the associated fingering dynamics and flow structure. We base our analysis on fully resolved numerical simulations under the Oberbeck–Boussinesq condition. The problem has five governing parameters: The salinity Prandtl number, Prs (momentum-to-salinity diffusivity ratio); the salinity Rayleigh number, Ras (measure of the fluid instability due to salinity differences); the Lewis number, Le (thermal-to-salinity diffusivity ratio); the density ratio, Λ (measure of the effective flow stratification), and the shear rate, Γ. Simulations are performed at fixed Prs, Ras, Le, and Λ, while the effect of shear is accounted for by considering different values of Γ. Preliminary results show that shear tends to damp the growth of fingering instability, leading to highly anisotropic DDC dynamics associated with the formation of regular salinity sheets. These dynamics result in significant modifications of the vertical transport rates, giving rise to negative diffusive fluxes of salinity and significant reduction of the total scalar transport, particularly of its convective part.

Deep-convective influence on the upper troposphere–lower stratosphere composition in the Asian monsoon anticyclone region: 2017 StratoClim campaign results

Abstract. The StratoClim stratospheric aircraft campaign took place in summer 2017 in Nepal (27 July–10 August) and provided for the first time a wide dataset of observations of air composition inside the Asian monsoon anticyclone (AMA). In the framework of this project, with the purpose of modelling the injection of pollutants and natural compounds into the stratosphere, we performed a series of diffusive back trajectory runs along the flights' tracks. The availability of in situ measurements of trace gases has been exploited to evaluate the capability of the trajectory system to reproduce the transport in the upper troposphere–lower stratosphere (UTLS) region. The diagnostics of the convective sources and mixing in the air parcel samples have been derived by integrating the trajectory output with high-resolution observations of cloud tops from the Meteosat Second Generation (MSG1) and Himawari geostationary satellites. Back trajectories have been calculated using meteorological fields from European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-Interim and ERA5) at 3 and 1 h resolution, using both kinematic and diabatic vertical motion. The comparison among the different trajectory runs shows, in general, a higher consistency with observed data as well as a better agreement between the diabatic and kinematic version when using ERA5-based runs with respect to ERA-Interim. Overall, a better capacity in reproducing the pollution features is finally found in the diabatic version of the ERA5 runs. We therefore adopt this setting to analyse the convective influence in the UTLS starting from the StratoClim observations. A large variety of transport conditions have been individuated during the eight flights of the campaign. The larger influence by convective injections is found from the continental sources of China and India. Only a small contribution appears to be originated from maritime regions, in particular the South Pacific and the Bay of Bengal, which, unexpectedly, was not particularly active during the period of the campaign. In addition, a mass of clean air injected from a typhoon has also been detected at around 18 km. Thin filamentary structures of polluted air, characterized by peaks in CO, are observed, mostly associated with young convective air (age less than a few days) and with a predominant South China origin. The analysis revealed a case of direct injection of highly polluted air close to the level of the tropopause (anomalies of around 80 ppbv injected at 16 km) that then kept rising inside the anticyclonic circulation. Due to the location of the campaign, air from continental India, in contrast, has been only observed to be linked to air masses that recirculated within the anticyclone for 10 to 20 d, resulting in a lower concentration of the trace gas. The analysis of a flight overpassing an intense convective system close to the southern Nepalese border revealed the injection of very young air (few hours of age) directly in the tropopause region (∼18 km), visible in the trace gases as an enhancement in CO and a depletion in the O3 one. From the whole campaign, a vertical stratification in the age of air is observed: up to 15 km, the age is less than 3 d, and these fresh air masses constitute almost the totality of the air composition. A transition layer is then individuated between 15 and 17 km, where the convective contribution is still dominant, and the ages vary between 1 and 2 weeks. Above this level, the mean age of the air sampled by the aircraft is estimated to be 20 d. There, the convective contribution rapidly decreases with height and finally becomes negligible around 20 km.

Buoyancy-driven melting and solidification heat transfer analysis in encapsulated phase change materials

Controlled melting and solidification in encapsulated phase change materials (PCMs) is of practical interest, for example, in latent heat storage applications. The choice of PCMs and the dynamic heat transfer characteristics during phase change - affecting the transient charging/discharging rates - are decisive for the energy and power density of the heat storage option. For example, highly conductive, high melting point (above 700 K) metal alloys have a potential advantage as a high energy and power density latent heat storage, compared to the widely used low conducting molten salt and paraffin wax. This advantage depends on the relative dominance of heat transfer modes that vary depending on the thermal properties of the PCM, shape of the encapsulation, and the load conditions, and must be quantified to warrant a fair comparison. We developed a 2D transient phase change model of encapsulated PCMs, accounting for phase change over a temperature range, volumetric expansion and contraction, and multi-mode heat transfer within the encapsulated PCM. The enthalpy-porosity method was used to model the phase change in a fixed grid. Validation was performed with literature data of low and high-conductivity PCM experiments (RT27 and lead). Two types of PCMs were subsequently investigated in detail: a high-conducting binary-eutectic alloy Al-12.6Si and a low-conducting commercially available RT27 paraffin wax (Rubitherm GmbH). The model was used to compare the phase change process and the strength of the various modes of heat transfer in the PCM filled cylindrical stainless-steel encapsulations in horizontal and vertical orientation with constant temperature walls. A full parameter study and non-dimensional analysis are presented for different PCMs. The results quantified the influence of boundary conditions, thermophysical properties and geometrical parameters on the phase change process and the contribution of the natural convection within the encapsulation. The non-dimensional analysis linked the melt fraction and heat transfer rates to a combination of the Fourier, Stefan, Rayleigh and Nusselt numbers. Fitting of the exponents of non-dimensional number groups to the heat transfer calculations allowed to provide general correlations for melting time, melt fraction, heat transfer rates, and characteristics of melting. Such correlations provide general understanding of the transient heat transfer in phase change media and provide engineering tools for designing, for example, a latent heat storage unit.

Contribution of thermocapillary convection to liquid evaporation

A series of 12 steady-state evaporation experiments were conducted in which water evaporated into a controlled environment from one of either two funnels that had similar geometry but were constructed of PMMA or stainless steel. We examined the energy flux by thermal conduction and thermocapillary convection. We compared it with total energy required to evaporate water at the rate measured with a syringe pump. The results indicate that for the PMMA funnel, thermal conduction fully supplied the energy required for evaporation at the observed rate, and that thermocapillary convection was negligible. However, for the stainless steel funnel, the measurements showed that thermal conduction supplied approximately 50% of the energy. Once the thermocapillary transport of energy along the interface using the previously reported value of surface thermal capacity was accounted for, conservation of energy was fully satisfied meaning that thermocapillary convection contributed approximately 50%. The results support the validity of the proposed surface thermal capacity for water and highlights the necessity of including the thermocapillary transport. We also evaluated the possibility of a sharp temperature variation in the vapor phase near the interface and found that it led to an over estimating the energy required for evaporation.

An experimental and numerical study of forced convection heat transfer from rectangular fins at low Reynolds numbers

This paper presents an experimental and numerical study of forced convection heat transfer from straight rectangular fins on a horizontal surface at low Reynolds numbers ranging from 2600 to 6800. Heat transfer for a fixed number of fins, fin spacing, and length was measured in a wind tunnel for varying inlet air velocity and heat input. Three-dimensional conjugate heat transfer simulations were also conducted to characterize important features of heat transfer and flow of the fin array and the effects of fin channel length on heat transfer. A maximum relative error of about 4.28% was found between the experimental and the numerical results for the average Nusselt number. The experimental results showed that forced convection heat transfer is characterized by an approximately linear relationship between the Nusselt and Reynolds numbers, which also agreed reasonably well with the available correlations for fully developed turbulent convective heat transfer for tubes with uniform heat flux. Further investigations of the effects of fin geometry were made numerically. Shorter fin channels, where the flow is thermally and hydrodynamically developing, were more effective in heat transfer than longer ones, where the flow approaches or reaches full development. It is shown that heat transfer rate per channel decreases linearly with the increase in channel length, but remains approximately constant with the increase in number of fins. It is also shown that the contributions of natural convection and radiation were negligible for the specific fin design and the range of inlet flow conditions and maximum fin temperature investigated here.

In-situ flame particle tracking based on barycentric coordinates for studying local flame dynamics in pulsating Bunsen flames

Flame particles (FP) are massless, virtual particles which follow material points on the flame surface. This work presents a tracking algorithm for FPs which utilizes barycentric coordinates. The methodology can be used with any cell shape in the computational mesh and allows computationally fast spatial interpolation as well as efficient determination of the intersection of FP trajectories with iso-surfaces. In contrast to previous flame particle tracking (FPT) approaches, the code is fully parallelized and can therefore be used in-situ during the simulation. It also includes fully parallelized computation of flame consumption speed by integrating reaction rates along a line normal to the flame surface at each FP position. Direct numerical simulations of laminar pulsating premixed hydrogen–air Bunsen flames serve as validation cases and showcase the added value of tracking material points for studying local flame dynamics. Exciting the inlet flow harmonically with frequencies equal to the inverse flame time scale leads to a pulsating mode where the flame front is corrugated. Ten times higher frequencies nearly resemble the steady state solution. The FPs are seeded along the flame surface and are used to track the unsteady diffusive, convective and chemical contributions at arbitrary points on the flame front over time. Their trajectories reveal a phase shift between the unsteady flame stretch rate and local flame speed of the order of 0.1 flame time scales for rich hydrogen flames. This is caused by a time delay between straining and stretch due to curvature. The reason is that diffusive processes follow the time signal of curvature while chemical processes are most strongly affected by the straining rate, which dominates the high Lewis number hydrogen flames investigated. This time history effect may help to explain the large scattering in the correlation of local flame speed with flame stretch found in turbulent flames.

Flow boiling heat transfer and pressure drop data of non-azeotropic mixture R455A in a horizontal 6.0 mm stainless-steel tube

Abstract This paper carries out an experimental investigation on R455A, a low GWP (148) non azeotropic mixture, to determine its performance in terms of flow boiling heat transfer coefficient and pressure drop in a smooth circular horizontal stainless-steel tube, with an internal diameter of 6 mm. The effect of the operating parameters, such as mass flux (from 300 to 500 kg‧m−2‧s−1), bubble saturation temperature (from 10 to 40 °C), heat flux (from 5 to 40 kW‧m−2) and vapor quality (between 0 and 1) is analyzed. Results show an increase of the heat transfer coefficient with mass and heat flux, while it decreases with saturation temperature. On the other hand, pressure drop increases with mass flux, and decreases with bubble saturation temperature. An assessment of several predictive methods from literature is finally conducted. Two-phase heat transfer methods based on an exclusive convective contribution can predict quite well the experimental results, while all other methods improve their statistics once a glide-related modification on the nucleate boiling contribution is assumed. The fictional pressure gradient results are well fitted with the separated flow model of Muller Steinhagen and Heck and with the Cicchitti two-phase viscosity expression considering a homogeneous flow approach.

Revisiting melting heat transfer of nano-enhanced phase change materials (NePCM) in differentially-heated rectangular cavities using thermochromic liquid crystal (TLC) thermography

The melting of nano-enhanced phase change materials (NePCM) in differentially-heated rectangular cavities was revisited by employing a novel indirect visualization method, i.e., the thermochromic liquid crystal (TLC) technique, to track the invisible phase interfaces. Detailed natural convective flow and heat transfer during melting were also predicted by numerical simulations based on the enthalpy-porosity method, which were validated by comparing with the TLC observations. Two height-to-width aspect ratios of the cavity, i.e., 0.8 and 1.25, were studied. Based on the observed evolutions of the phase interface, it was shown that natural convection originating from the upper corner nearby the phase interface causes the difference in melting patterns between the upper and lower parts of the cavity. The decelerated melting in the presence of NePCM samples with higher loadings (and hence much greater viscosity) was confirmed intuitively by the more vertical and flatten phase interfaces captured by TLC, indicating that heat conduction becomes the dominant mode of heat transfer during melting as a result of the significantly deteriorated natural convection effect. Moreover, when the cavity becoming more slender, the advantage of increased thermal conductivity could be more sufficiently taken because the shorter characteristic length inherently lowers the contribution of natural convection.

Experimental and numerical investigations of a modified designed baseboard radiator using an air gap enhancing free convection heat transfer

The present study aims to improve the thermal performance of conventional radiant baseboard heating systems by proposing a novel and modified design with the least cost and complexity. The convection to radiation ratio for conventional and modified baseboard designs is compared by several experimental tests and numerical simulations of heat transfer between internal pipe flow and indoor air convective flow. By examining the temperature and velocity profiles of both radiant baseboards, it is revealed that the total thermal power of the modified radiant baseboard is 34% higher than that of the conventional one, especially at the occupied zone which has raised indoor air temperature by up to 2 °C in different resident postures such as standing, sitting and sleeping. Also, the results show that the convection contribution of the modified design of radiant baseboard is greater than the conventional one, more than 45% in the same conditions, while radiation contribution also grows considerably. The results of 3D steady-state numerical simulations show that heat output from the modified radiant baseboard per cm of panel height is 3 times more than that of the conventional one. Consequently, it is indicated that the modified radiant baseboard has a lower supply temperature, leading to energy-space saving, while its capability to overcome cold air down draught flow is more, which can be of interest to experts in the thermal comfort field. Furthermore, by choosing a modified radiant baseboard, the required length of the baseboard will be reduced by 50% compared to the conventional one, and this can have a significant impact on the architectural and economic limitations of the building industry.

The Influence of Convective Momentum Transport and Vertical Wind Shear on the Evolution of a Cold Air Outbreak

AbstractTo study the influence of convective momentum transport (CMT) on wind, boundary layer and cloud evolution in a marine cold air outbreak (CAO) we use large‐eddy simulations subject to different baroclinicity (wind shear) but similar surface forcing. The simulated domain is large enough, km2), to develop typical mesoscale cellular convective structures. We find that a maximum friction induced by momentum transport (MT) locates in the cloud layer for an increase of geostrophic wind with height (forward shear, FW) and near the surface for a decrease of wind with height (backward shear, BW). Although the total MT always acts as a friction, the interaction of friction‐induced cross‐isobaric flow with the Coriolis force can develop supergeostrophic winds near the surface (FW) or in the cloud layer (BW). The contribution of convection to MT is evaluated by decomposing the momentum flux by column water vapor and eddy size, revealing that CMT acts to accelerate subcloud layer winds under FW shear and that mesoscale circulations contribute significantly to MT for this horizontal resolution (250 m), even if small‐scale eddies are nonnegligible and likely more important as resolution increases. Under FW shear, a deeper boundary layer and faster cloud transition are simulated, because MT acts to increase surface fluxes and wind shear enhances turbulent mixing across cloud tops. Our results show that the coupling between winds and convection is crucial for a range of problems, from CAO lifetime and cloud transitions to ocean heat loss and near‐surface wind variability.

Near-surface mean and gust wind speeds in ERA5 across Sweden: towards an improved gust parametrization

The ERA5 reanalysis product has been compared with hourly near-surface wind speed and gust observations across Sweden for 2013–2017. ERA5 shows closer agreement than the previous ERA-Interim reanalysis with regard to both mean wind speed and gust measurements, although significant discrepancies are still found for inland and mountainous regions. Therefore, attempts have been made to improve formulations of the gust parametrization used in ERA5 by adding an elevation-dependency and by adjusting the convective gust contribution. Major improvements, especially over mountain regions, are achieved when the elevation differences among the stations are considered. Closer agreement between the observed and parametrized gusts is reached when the convective gust contribution is also tuned. The newly designed gust parametrization was also tested for Norway, which is characterized by more complex topography. Wind gusts from the selected Norwegian stations are more realistically simulated when both the elevation-dependency and the tuned convective contribution are implemented, although the parametrized gusts are still negatively biased. Such biases are not explained by the different in gust duration in recorded wind gusts between Sweden and Norway.

Wall heat flux partitioning during subcooled flow boiling of NaCl solutions and pure water

A careful evaluation on the wall heat flux partitioning is of great importance for the deep understanding the nucleate flow boiling heat transfer to saline solution. In this work, the portion of each heat transfer contribution was analyzed through wall heat flux partitioning model after model validation. The key parameters used in the model were discussed and the reasonable values were determined. The experimental data of bubble departure diameter, departure frequency, active nucleation site density, bubble waiting time and growth time, were combined into the model to eliminate the uncertainties of empirical closure sub-models. The comparison of heat flux partitioning of subcooled flow boiling between NaCl solution and pure water was presented under different operating conditions. The results showed that for the operating conditions of Qw > 180 kW/m2 or G < 400 kg/(m2·s), the contribution of single-phase liquid convection in the heat transfer of saline solution is greater than that in pure water, due to the bigger bubble diameter and more intensive bubble-induced turbulence. Quenching heat flux for NaCl solution is lower than that in pure water, owing to the smaller bubble departure frequency. For the lower heat flux or higher mass flux conditions, however, the contrary trend was observed. The dominant heat transfer mechanisms for solution and pure water during subcooled nucleate flow boiling were revealed. The interactions between bubble dynamic and heat transfer characteristics were carefully investigated.

Performance evaluation on the gradient design of pore parameters for metal foam and pin fin-metal foam hybrid structure

This paper reported a study on the heat transfer in solidification of phase change materials (PCMs) embedded in metal foam. Heat transfer enhancement techniques are addressed in solidification including the insertion of pin fins and gradient design of pore parameters. Numerical models to describe the transient phase change heat transfer are established through the volume-averaged theory. One-temperature model is accounted for under the core assumption of local thermal equilibrium state. An experimental test rig is designed and established to verify the feasibility of the built numerical models by means of comparing solidification fronts and temperatures at PCM. The effects of gradients in parent materials, porosity and pore density for metal foam upon solidification behavior in metal foam and pin fin-metal foam hybrid structures are quantified. The contribution of local natural convection to the solidification behavior is justified and found this effect can be safely neglected for simulation on solidification in metal foam. Results demonstrate that the insertion of pin fins notably improve the solidification in metal foam regardless of gradient in pore parameters. The gradient in porosity rather than parent materials for the pin fin-metal foam hybrid structures can further improve the solidification rate. As for metal foam, both gradient in parent materials and graded porosity can significantly promote the solidification. The best heat transfer structure is recommend to be a pin fin-metal foam hybrid structure with gradient in metal foam porosity, outperforming other competing heat transfer techniques including pin fins or metal foam.

Reflective insulation assemblies for above-ceiling applications

Previously published hot-box data have been used to construct equations for the thermal resistance of enclosed reflective air spaces (reflective insulation assemblies) for a wide range of temperatures, air gap dimensions, thermal emittances, and heat flow directions. The thermal resistances or R-values (RSI) calculated with the equations compare favorably with previously published thermal resistances. Significant differences from RSI values (m2 K/W) calculated using ISO 6946 were observed. Equations for calculating heat transfer coefficients for conduction–convection with constants for the heat flow directions up, 45° up, horizontal, 45° down, and down are contained in this article. The conduction–convection coefficient for planar air spaces oriented at any angle and heated above can be obtained by interpolation between heat flow down and heat flow at a downward angle of 45° or heat flow down at an angle of 45° and horizontal heat flow. The overall heat transfer coefficient is obtained by adding the thermal radiation contribution to the conduction–convection contribution. The RSI of enclosed reflective air spaces is the reciprocal of the overall heat transfer coefficient for the air space. This air space RSI is especially useful as input for the calculation of U-values for ceiling–roof assemblies located in hot climates.

Development of a Scalable Two-Additive Electrodeposition System for Bottom-up Copper Filling of Sub-Millimeter through Silicon Vias

A two-additive electrodeposition system for Cu feature filling of sub-millimeter scale through silicon vias (TSVs) is discussed. This 1.0 mol/L CuSO4 – 0.5 mol/L H2SO4 acid sulfate electrolyte contains a polyether surfactant and halide additive which together promote a bottom-up Cu growth profile through positive feedback between localized disruption of the adsorbed polyether-halide layer and metal deposition.1 The functionality of this system relies on a tightly controlled additive concentration gradient down the length of the feature at the onset of metal deposition to allow bottom-up Cu growth to propagate. Careful consideration of convective contributions is also made to ensure an appropriate boundary layer thickness is maintained for non-linear bottom-up filling, where localized active and passive zone bifurcation of rapid electrodeposition is present. Initial experiments demonstrate the efficacy of potentiostatic control in determining the applied current window by which galvanostatic control can ultimately be achieved for a sub-millimeter feature geometry. Nominally void-free bottom-up deposition of multiple recessed feature geometries is presented.One advantage of the two-additive bottom-up electrodeposition system is that it has been demonstrated across multiple length scales and with multiple materials,2-9 making it suitable for heterogeneous integration (HI) of a wide array of devices which can require unique feature geometries and interconnect materials. Depending on the application, the two-additive bottom-up electrodeposition system can replace the more traditional three or four-additive system in damascene processing10 which can yield superconformal deposition profiles in TSVs as explained by the curvature enhanced adsorbate coverage (CEAC) mechanism.11 The CEAC mechanism relies upon high curvature areas and is thus most appropriate for micro and nanoscale features which exhibit proximity of locally high curvature areas as deposition proceeds due to area change affects. Void-free superconformal deposition is more difficult to repeatably achieve on larger, mesoscale features. Thus, a bottom-up deposition technique that relies upon the non-curvature dependent S-shaped negative differential resistance (S-NDR) between just two constituent additives to fill conductively seeded features is desirable.The issue of scalability is of significant interest after developing an electrodeposition process at the die level. While this two-additive system has been successfully implemented using potentiostatic control, industrial full wafer plating tools rely exclusively on galvanostatic control as they are not equipped with reference electrodes. In addition, they combine wafer rotation and forced fluid flow to maintain a controlled convective contribution across the wafer. So, to demonstrate scalability of this two-additive chemistry, the effect of the field surface area, feature geometry, and sample rotation rate on the deposition profile at a given applied potential or current is investigated. Altering the feature quantity and field area for a given deposition experiment can provide a means to estimate the effective current density required to fill features at the same rate across different sample sizes. By altering the sample rotation rate in the otherwise quiescent electrolyte, the impact of the transport phenomena of the constituent surfactant and halide additives are assessed. Cross-sectional images along with computed tomography (CT) produced images are produced to obtain Cu fill profiles across features arrays to demonstrate TSV uniformity. 1. P. Moffat and D. Josell, J. Electrochem. Soc., 159, D208 (2012).2. A. Menk et al., J. Electrochem. Soc. 166, D3066-D3071 (2018).3. Josell, M. E. Williams, S. Ambrozik, C. Zhang, T. P. Moffat, J. Electrochem. Soc. 166, D487-D495 (2019).4. Josell, M. Silva, T. P. Moffat, ECS Trans. 75, 25-30 (2016).5. Josell, T. P. Moffat, J. Electrochem. Soc. 166, D3022-D3034 (2018).6. Josell, T. P. Moffat, ECS Trans. 75, 19-24 (2016).7. Josell, T. P. Moffat, J. Electrochem. Soc. 162, D129-D135 (2015).8. Josell, T. P. Moffat, J. Electrochem. Soc. 160, D3035-D3039 (2013).9. Josell, L. A. Menk, A. E. Hollowell, M. Blain, T. P. Moffat, J. Electrochem. Soc. 166, D3254-D3258 (2019).10. C. Andricacos, C. Uzoh, J. O. Dukovic, J. Horkans, and H. Deligianni, IBM J. Res. Dev., 42, 567 (1998).11. P. Moffat, D. Wheeler, M. Edelstein, and D. Josell, IBM J. Res. Dev., 49, 19 (2005). Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.

Effect of thermocapillary convection on the melting of phase change materials in microgravity: Experiments and simulations

Experiments on the melting of n-octadecane in the presence of a free surface were performed under the reduced gravity conditions of a parabolic flight, where natural gravitational convection is negligible. The results constitute the first systematic experimental investigation of the capacity of thermocapillary (Marangoni) convection to enhance heat transfer during the melting of organic Phase Change Materials (PCMs) in microgravity, where their low thermal conductivity leads to significant lag (long response times) that, in some applications, constrains their utility as passive thermal control devices. Rectangular containers of three different aspect ratios holding n-octadecane and a layer of air were heated from one side in order to observe the melting process. During the relatively short microgravity periods, which lasted approximately 20 s, the effect of thermocapillary convection remained concentrated in a layer near the free surface that is thinner than the PCM height H for the aspect ratios considered. The weak dependence of this layer on the lower region, where heat transport is dominated by thermal diffusion, means that the relative contribution of thermocapillary convection decreases with H. The experimental results are supported and complemented by a series of numerical simulations modelling the phase change and liquid dynamics with an enthalpy-porosity-based formulation of the Navier-Stokes equations. Very good agreement with the experimental observations is found.