CTRC Research Publications

Design of Molten-Salt Thermocline Tanks for Solar Thermal Energy Storage

S. M. Flueckiger et al. — Thu, 18 Apr 2013 22:36:06 PDT

Molten-salt thermocline tanks are a low-cost option for thermal energy storage in concentrating solar power systems. A review of previous experimental and numerical thermocline tank studies is performed to identify key issues associated with tank design and performance. Published models have shown that tank discharge performance improves with both larger tank height and smaller internal filler diameter due to increased thermal stratification and sustained outflow of molten salt with high thermal quality. For well-insulated (adiabatic) tanks, low molten-salt flow rates reduce the axial extent of the heat-exchange region and increase discharge efficiency. Under nonadiabatic conditions, low flow rates become detrimental to stratification due to the development of fluid recirculation zones inside the tank. For such tanks, higher flow rates reduce molten-salt residence time inside the tank and improve discharge efficiency. Despite the economic advantages of a thermocline tank, thermal ratcheting of the tank wall remains a significant design concern. The potential for thermal ratcheting is reduced through the inclusion of an internal thermal insulation layer between the molten salt and tank wall to diminish temperature oscillations along the tank wall. Future research directions are also pointed out, including combined analyses that consider the solar receiver and power generation blocks as well as optimization between performance and economic considerations.

Evaporation Analysis of Sintered Wick Microstructures

K. K. Bodla et al. — Wed, 27 Mar 2013 22:06:05 PDT

Heat pipes offer passive transport of heat over long distances without incurring a significant drop in temperature. Topological and microstructural details of the wick material embedded in a heat pipe help determine its thermal performance. A good understanding of pore-scale transport phenomena is crucial to enhancing heat pipe performance. In this study, pore-scale analysis of thin-film evaporation through sintered copper wicks is performed. X-ray microtomography is employed to generate geometrically faithful, feature-preserving meshes. Commercial sintered wicks with particle sizes in the range of 45–60 lm, 106–150 lm and 250–355 lm and with approximately 61% porosity are considered. The capillary pressure, characteristic pore radius, percentage thin film area and evaporative mass and heat fluxes are computed using a volume of fluid (VOF) approach. Two different solution strategies are employed to stabilize the numerical solution and to improve convergence. After verifying that these strategies yield the correct solution, the VOF model is used to obtain static meniscus shapes in the pore space of the sintered wick samples. The meniscus shape is then held fixed and steady-state, thin-film evaporation analysis is performed. Liquid–vapor phase change heat transfer is modeled using a modified Schrage equation. Based on the present analysis, the best performing sample (particle size range) is identified along with the optimum contact angle.

Heat Transfer and Flow Fields in Confined Jet Impingement

S. V. Garimella — Fri, 22 Mar 2013 13:47:55 PDT

Confined jet impingement of air and different liquids is reviewed. The influence of confinement on the heat transfer and fluid mechanics of governing parameters such as jet diameter and geometry, Reynolds number, jet-to-target separation distance, exit velocity profile and turbulence levels, fluid properties, and target size is discussed. This review deals primarily with turbulent jets. Single- and multiple-jet impingement are considered, including predictive correlations for stagnation-point and area-averaged heat transfer. The velocity and turbulence fields in confined jet impingement are also described, with particular reference to their effect on the heat transfer obtained. The important role of flow visualization in understanding confined flow fields is demonstrated. Augmentation of jet impingement heat transfer by employing surface enhancements is also considered. The review includes discussions of numerical modeling efforts.

Technological Drivers in Data Centers and Telecom Systems: Multiscale Thermal, Electrical, and Energy Management

S. V. Garimella et al. — Thu, 14 Mar 2013 17:14:46 PDT

including thermal, electrical and energy management challenges, based on discussions at the 2nd Workshop on Thermal Management in Telecommunication Systems and Data Centers in Santa Clara, California, on April 25–26, 2012. The relevance of thermal management in electronic systems is reviewed against the background of the energy usage of the information technology (IT) industry, encompassing perspectives of different sectors of the industry. The underlying drivers for progress at the business and technology levels are identified. The technological challenges are reviewed in two main categories – immediate needs and future needs. Enabling cooling techniques that are currently under development are also discussed.

Metal Functionalization of Carbon Nanotubes for Enhanced Sintered Powder Wicks

A. S. Kousalya et al. — Fri, 15 Feb 2013 12:37:03 PST

Phase change cooling schemes involving passive heat spreading devices, such as heat pipes and vapor chambers, are widely adopted for thermal management of high heat-flux technologies. In this study, carbon nanotubes (CNTs) are fabricated on a 200 micrometer thick sintered copper powder wick layer using microwave plasma enhanced chemical vapor deposition technique. A physical vapor deposition process is used to coat the CNTs with a varying thickness of copper to promote surface wetting with the working fluid, water. Thermal performance of the bare sintered copper powder sample (without CNTs) and the copperfunctionalized CNT-coated sintered copper powder wick samples is compared using an experimental facility that simulates the capillary fluid feeding conditions of a vapor chamber. A notable reduction in the boiling incipience superheat is observed for the nanostructured samples. Additionally, nanostructured samples having a thicker copper coating provided a considerable increase in dryout heat flux, supporting heat fluxes up to 457 W/cm^2 from a 5 mm x 5 mm heat input area, while maintaining lower surface superheat temperatures compared to a bare sintered powder sample; this enhancement is attributed primarily to the improved surface wettability. Dynamic contact angle measurements are conducted to quantitatively compare the surface wetting trends for varying copper coating thicknesses and confirm the increase in hydrophilicity with increasing coating thickness.

Modeling and Design Optimization of Ultra-Thin Vapor Chambers for High Heat Flux Applications

R. Ranjan et al. — Wed, 16 Jan 2013 09:38:53 PST

Passive phase-change thermal spreaders, such as vapor chambers have been widely employed to spread the heat from small-scale high-flux heat sources to larger areas. In this paper, a numerical model for ultrathin vapor chambers has been developed, which is suitable for reliable prediction of the operation at high heat fluxes and small scales. The effects of boiling in the wick structure on the thermal performance are modeled, and the model predictions are compared with experiments on custom-fabricated vapor chamber devices. The working fluid for the vapor chamber is water and a condenser side temperature range of 293 K–333 K is considered. The model predictions agree reasonably well with experimental measurements and reveal the input parameters to which thermal resistance and vapor chamber capillary limit are most sensitive. The vapor space in the ultrathin devices offers significant thermal and flow resistances when the vapor core thickness is in the range of 0.2 mm–0.4 mm. The performance of a 1-mm-thick vapor chamber is optimized by studying the variation of thermal resistance and total flow pressure drop as functions of the wick and vapor core thicknesses. The wick thickness is varied from 0.05 to 0.25 mm. Based on the minimization of a performance cost function comprising the device thermal resistance and flow pressure drop, it is concluded that the thinnest wick structures (0.05 mm) are optimal for applications with heat fluxes below 50 W/cm2, while a moderate wick thickness of 0.1 mm performs best at higher heat flux inputs (>50 W/cm2).

Cyclic Operation of Molten-Salt Thermal Energy Storage in Thermoclines for Solar Power Plants

Z. Yang et al. — Tue, 15 Jan 2013 21:26:57 PST

The cyclic operation of molten-salt thermal energy storage thermoclines for solar thermal power plants is systematically investigated. A comprehensive, two-temperature model is first developed for the cyclic operation of a thermocline operating with a commercially available molten salt as the heat transfer fluid and quartzite rock as the filler. Volume-averaged mass and momentum equations are employed, with the Brinkman–Forchheimer extension to the Darcy law used to model the porous-medium resistance. Energy equations for the molten salt and the filler are coupled by an interstitial Nusselt number representing the heat transfer between the phases. A finite-volume approach is employed to solve the governing equations. The model is validated against experiments from the literature and then used to systematically study the cyclic behavior of the thermocline thermal storage system. Thermal characteristics including temperature profiles and cycle efficiency are explored. Guidelines are developed for designing the dimensions and molten salt flow rates for solar thermocline systems of different power capacities. The cycle efficiency is found to be improved at smaller melt Reynolds numbers, larger length ratios (molten salt flow distance in a half-cycle to the filler particle diameter) and larger tank heights. The filler particle diameter and the tank volume are found to strongly influence the cycle efficiency.

Nucleate Boiling from Smooth and Rough Surfaces--Part 1: Fabrication and Characterization of an Optically Transparent Heater-Sensor Substrate with Controlled Surface Roughness

J. P. McHale et al. — Sat, 17 Nov 2012 18:32:07 PST

devised to conduct detailed heat transfer and bubble measurements during boiling on a heater surface with controlled roughness. This first of two companion papers discusses details of the fabrication, construction, and operation of the experimental facility. Test pieces are fabricated from 50.8 mmx50.8 mmx3.18 mm borosilicate glass squares that are roughened by abrading with diamond compound with particles of known size and then annealed in order to control small-scale roughness features. Heater/sensor devices are fabricated by depositing indium tin oxide (ITO), a transparent electrically conductive material, on top of the roughened glass surfaces. A method for calibrating the ITO devices for use as temperature sensors is developed. Experimental boiling curves are reported for seven surfaces of different roughness tested in the perfluorinated fluorocarbon liquid, FC-72. The measured receding contact angle for FC-72 on the ITO-coated surfaces is reported. The wall superheat during saturated pool boiling at atmospheric pressure was found not to vary consistently with surface average roughness (Ra) values. Qualitative differences between smooth and rough surface boiling visualizations obtained simultaneously from the side and from below the heaters are discussed.

Nucleate Boiling from Smooth and Rough Surfaces--Part 2: Analysis of Surface Roughness Effects on Nucleate Boiling

J. P. McHale et al. — Sat, 17 Nov 2012 18:11:07 PST

The effect of surface roughness on nucleate boiling heat transfer is not clearly understood. This study is devised to conduct detailed heat transfer and bubble measurements during boiling on a heater surface with controlled roughness. This second of two companion papers presents an analysis of heat transfer and bubble ebullition in nucleate boiling with new measures of surface roughness: area ratio, surface mean normal angle, and maximum idealized surface curvature. An additional length scale of importance, the maximum base diameter of an emergent bubble, is identified. Measurements of bubble departure diameters, growth periods, ebullition periods, and void fraction above the surface are obtained from high-speed videographic visualizations by an automated procedure. Correlations of heat transfer coefficient and bubble ebullition characteristics with different measures of surface roughness are compared in terms of relative uncertainty. The data set of results for pool boiling in the perfluorinated dielectric liquid, FC-72, are found to correlate best with a length-scale filtered value of average roughness Ra, filt. Over a larger database with three different data sets including FC-72, FC-77, and water at atmospheric pressure, the most reliable correlations were obtained with the appropriately filtered area ratio. FC-72 bubble growth curves are well correlated for all test conditions with the normalized relationship D*~(t*)^1/3. Finally, the maximum void fraction in the region above the surface is correlated with normalized heat flux for these data and for water as the two-thirds power of heat flux.

Thermodynamic and Kinetic Investigation of a Chemical Reaction-Based Miniature Heat Pump

Scott M. Flueckiger et al. — Sat, 10 Nov 2012 10:21:41 PST

Representative reversible endothermic chemical reactions (paraldehyde depolymerization and 2-proponal dehydrogenation) are theoretically assessed for their use in a chemical heat pump design for compact thermal management applications. Equilibrium and dynamic simulations are undertaken to explore the operation of the heat pump which upgrades waste heat from near room temperature by approximately 20 degrees in a minimized system volume. A model is developed based on system mass and energy balances coupled with kinetic equations to ascertain mixture conditions at each state point in the loop, as well as mass flow rate, minimum work input, and minimum endothermic reactor volume according to defined reservoir temperatures and desired heat load. Assuming that a pervaporation process is employed in both reaction systems to achieve the requisite mixture compositions for sustained operation, the simulations show that the chemical heat pump can pump 5Wof power with endothermic reactor volumes of as little as 60–93 cm3, depending on the selected chemical reaction. Low exergy efficiencies remain a significant design consequence, but the system performance in terms of environmental impact and COP are comparable with, and in some cases better than, the performance of alternative technologies under the same conditions.

Evaporative Heat Transfer from an Electrowetted Liquid Ribbon on a Heated Substrate

Chris P. Migliaccio et al. — Sat, 10 Nov 2012 09:09:23 PST

Evaporation of narrow water ribbons (of 5 and 7 lL volume) formed on a heated surface is investigated. Chemical and structural patterning of a silicon substrate is employed to fabricate a hydrophilic stripe that bisects hydrophobic pillar arrays of varied geometric roughness. Electrical heating of a 100 nm titanium layer on the back side of the device provides a constant heat flux. In the absence of electrical actuation, water introduced onto the substrate takes a bulging ribbon shape that is constrained to the immediate vicinity of the hydrophilic stripe. Electrowetting of the water ribbon spreads it into the hydrophobic pillar arrays on either side, leading to significant increases in maximum wetted width (up to 200%) and wettability (up to 80% reduction in contact angle). Infrared thermography is employed to characterize the cooling effect due to the spreading of the ribbon, while a goniometer monitors the ribbon shape. The heat transfer in each case is estimated through an energy balance analysis, and the results are compared with other electrowetting-based cooling techniques.

Thermal Management Challenges in Telecommunication Systems and Data Centers

S. V. Garimella et al. — Wed, 29 Aug 2012 20:06:36 PDT

The framework for this paper is the growing concern about the worldwide increasing energy consumption of telecommunications systems and data centers, and particularly the contribution of the thermal management system. The present energy usage of these systems is discussed, as well as the relationship between cooling system design and the total cost of ownership. This paper identifies immediate and future thermal bottlenecks facing the industry, ranging from technological issues at the component and system level to more general needs involving reliability, modularity, and multidisciplinary design. Based on this enumeration, the main challenges to implementing cooling solutions are reviewed. Particular attention is paid to implementing liquid cooling, since this technology seems the most promising to addressing the key thermal bottlenecks, and improving the future sustainability of thermal management in the telecom and data center industry. Finally, an outlook is presented toward future potential challenges.

Dissipative Forces in the eElectrowetted Cassie-Wenzel Transition on Hydrophobic Rough Surfaces

C. P. Migliaccio et al. — Wed, 29 Aug 2012 15:20:17 PDT

Dissipative forces in the electrowetting-induced Cassie-Wenzel transition on hydrophobic rough surfaces are explored. High-speed imaging of droplet shape evolution during the elec- trically induced spreading process allows for the location of the contact line to be tracked as a function of time. A surface energy analysis quantifies the total energy dissipated via nonconservative forces during the spreading process. Though identified as the dominant dissipative effect in droplet spreading on smooth surfaces, contact line friction is shown to have a relatively weak influence on the spreading on rough surfaces. Supplemental files are available for this article. Go to the publisher’s online edition of Nanoscale and Microscale Thermophysical Engineering to view the free supplemental file.

Frequency-dependent transient response of an oscillating electrically actuated droplet

S. Dash et al. — Tue, 28 Aug 2012 21:46:04 PDT

The transient response of a millimeter-sized sessile droplet under electrical actuation is experimentally investigated. Under dc actuation, the droplet spreading rate increases as the applied voltage is increased due to the higher electrical forces induced. At sufficiently high dc voltages, competition between the electrical actuation force, droplet inertia, the retarding surface tension force and contact line friction leads to droplet oscillation. The timescale for the droplet to attain its maximum wetted diameter during step actuation is analyzed. Systematic experiments are conducted over a frequency range of 5–200 Hz and actuation voltages of 40–80 Vrms to determine the dependence of droplet oscillation on these parameters. The response of the droplet to different actuation frequencies and voltages is determined in terms of its contact angle and contact radius variation. The frequency of the driving force (equal to twice the frequency of the applied electrical signal) determines the mode of oscillation of the droplet which, together with its resonance characteristics, governs whether the droplet contact angle and contact radius vary in phase or out of phase with each other. In addition to the primary frequency response at the electrical forcing frequency, the droplet oscillation exhibits sub-harmonic oscillation at half of the forcing frequency that is attributed to the parametric nature of the electrical force acting on the triple contact line of the droplet.

Multi-objective Optimization of Sustainable Single-Effect Water/Lithium Bromide Absorption Cycle

B. H. Gebreslassie et al. — Sat, 12 May 2012 21:46:05 PDT

A rigorous mathematical approach is developed for optimization of sustainable single-effect water/ Lithium Bromide (LiBr) absorption cooling cycles. The multi-objective formulation accounts for minimization of the chiller area as well as the environmental impact associated with the operation of the absorption cycle. The environmental impact is quantified based on the global warming potential and the Eco-indicator 99, both of which follow principles of life cycle assessment. The design task is formulated as a bi-criterion non-linear programming problem, the solution of which is defined by a set of Pareto points that represent the optimal compromise between the total area of the chiller and global warming potential. These Pareto sets are obtained via the epsilon constraint method. A set of design alternatives are provided for the absorption cycles rather than a single design; the best design can be chosen from this set based on the major constraints and benefits in a given application. The proposed approach is illustrated design of a typical absorption cooling cycle.

Second-Law Analysis of Molten-Salt Thermal Energy Storage in Thermoclines

S. Flueckiger et al. — Wed, 09 May 2012 21:02:58 PDT

The cyclic operation of a molten-salt thermocline tank is simulated to investigate the influence of internal granule diameter and external convection losses on tank performance. Practical constraints limiting thermocline tank height are taken into account. The authors two-temperature model, developed in earlier work (Solar Energy, 84, 974–985, 2010) for the analysis of heat transfer and fluid flow in the thermocline tank, is extended to monitor entropy generation and exergy transport. Storage performance is measured in terms of first- and second-law efficiency definitions, as well as a first-law efficiency used in conjunction with an outflow temperature criterion. Reducing the diameter of the fillerbed granules improves the thermocline tank performance by sustaining higher molten-salt outflow temperatures throughout the discharge phase of the cycle, which results in greater operational efficiency. External convection losses strongly influence entropy generation inside the tank fillerbed due to the development of radial temperature gradients and increased irreversible thermal diffusion. Convection losses also result in lower tank efficiencies due to the reduction of hot molten salt available inside the tank. A comparison of the different efficiency definitions employed in this work reveal that the ad hoc outflow temperature criterion used in past studies provides an overly conservative assessment of thermocline performance.

Visualization of Vapor Formation Regimes during Capillary-Fed Boiling

J. A. Weibel et al. — Tue, 08 May 2012 21:57:04 PDT

The current study investigates capillary-fed boiling of water from porous sintered powder wicks used in emerging high-effective-conductivity vapor chamber heat spreaders intended for management of hot spots with heat fluxes exceeding 500Wcm-2. Characterization of 1 mm thick wicks composed of 100 lm sintered copper particles is performed in a test facility which replicates the capillary feeding conditions that occur in such devices. Boiling curves are obtained for a 5 mm x 5 mm heated input area, along with high-speed in-situ visualization of the evaporation/boiling processes. Understanding the vapor formation regimes is essential to predictive modeling of the observed characteristics. Schematic representations of such regimes along the boiling curves are presented for homogeneous and modified wick structures. In general, incipience of boiling in sintered-powder wicks reduces the effective thermal resistance and, for small heat input areas, does not cause liquid starvation due to a capillary limitation. The thermal performance enhancement provided by two different augmentation methods is quantified and explained in terms of the observed vapor formation characteristics. Patterns fabricated within the sintered powder create multi-scale wicks with regions of different pore size. These patterns reduce thermal resistance throughout the boiling regime by increasing the permeability to vapor exiting the wick, as confirmed by visualization of the preferential vapor venting from the surface. At the highest heat fluxes investigated prior to dryout, a thin liquid film is observed to form in the recessed patterned areas at the base of the wick. Integration of copper-coated carbon nanotubes on to the sintered powder reduces the required superheat for boiling incipience, thus reducing the overall thermal resistance at low heat fluxes. Evaporation and boiling regime heat transfer predictions from several available correlations are compared to the current results, and are shown to corroborate the conclusions regarding vapor permeability.

Visualization of Vapor Formation Regimes during Capillary-Fed Boiling

J. A. Weibel et al. — Tue, 08 May 2012 21:52:36 PDT

Flow Regime-Based Modeling of Heat Transfer and Pressure Drop in Microchannel Flow Boiling

T. Harirchian et al. — Tue, 08 May 2012 21:45:16 PDT

Local heat transfer coefficients and pressure drops during boiling of the dielectric liquid fluorinert FC-77 in parallel microchannels were experimentally investigated in recent work by the authors. Detailed visu- alizations of the corresponding two-phase flow regimes were performed as a function of a wide range of operational and geometric parameters. A new transition criterion was developed for the delineation of a regime where microscale effects become important to the boiling process and a conventional, macroscale treatment becomes inadequate. A comprehensive flow regime map was developed for a wide range of channel dimensions and experimental conditions, and consisted of four distinct regions – bubbly, slug, confined annular, and alternating churn/annular/wispy-annular flow regimes. In the present work, phys- ics-based analyses of local heat transfer in each of the four regimes of the comprehensive map are formu- lated. Flow regime-based models for prediction of heat transfer coefficient in slug flow and annular/ wispy-annular flow are developed and compared to the experimental data. Also, a regime-based predic- tion of pressure drop in microchannels is presented by computing the pressure drop during each flow regime that occurs along the microchannel length. The results of this study reveal the promise of flow regime-based modeling efforts for predicting heat transfer and pressure drop in microchannel boiling.

The Importance of Turbulence during Condensation in a Horizontal Circular Minichannel

E. da Riva et al. — Tue, 08 May 2012 21:31:52 PDT

Three-dimensional simulations of condensation of refrigerant R134a in a horizontal minichannel are presented. Mass fluxes ranging from 50kgm-2 s-1 up to 1000kgm-2 s-1 are considered in a circular minichannel of 1mm diameter, and uniform wall and vapour–liquid interface temperatures are imposed as boundary conditions. The Volume of Fluid (VOF) method is used to track the vapour–liquid interface; the effects of interfacial shear stress, gravity and surface tension are taken into account. The influence of turbulence in the condensate film is analysed and compared against the assumption of laminar condensate flow by employing different computational approaches and validating the results against experimental data. Under the assumption of laminar condensate flow, experimental heat transfer coefficient values at low mass fluxes can be predicted, but the computed heat transfer coefficient is found to be almost independent of mass flux and vapour quality. Only when turbulence in the condensate film is taken into account does the numerical model capture the influence of mass flux that is observed in the experimental measurements.