LINK to Repository: https://doi.org/10.1016/j.ijft.2023.100333
ABSTRACT
The suitability of industrially significant synthetic oils with dispersed polymeric chains that can be used as dielectric coolants with enhanced heat transfer properties in single-phase immersion cooling for electric vehicle components is evaluated via molecular dynamics simulations (MD). The fluids investigated are a synthetic solvent poly-alpha-olefin (PAO-2) and a solution based on PAO-2 with a single olefin co-polymer (OCP) chain dissolved. The simulation model accurately predicts the experimental thermodynamic properties of PAO-2. The effect of the polymer chain on the structural behaviour of the solution and its relation with the rheological properties is predicted and analysed at various temperatures in the range of 293 K–373 K. It is found that polymer solution shows an average viscosity enhancement of 9.2% and thermal conductivity enhancement of 2% within the temperature range. These properties eventually influence the Weissenberg and Nusselt numbers that impact the heat transfer. Analysis of the hydrodynamic radius of PAO-2 molecules shows that OCP chemistry acts as a thickening agent in the solution. Addition of the polymer chain is also shown to accelerate the shear thinning process due to increase in storage and loss moduli. The terminal relaxation time of OCP decreases with temperature and shear rate. The work conclusively establishes the impact of molecular interactions of the weakly viscoelastic liquids on their macroscopic behaviour. The viscoelastic nature of the examined polymer solution can lead to vortex roll-up in constricted flows inducing heat transfer enhancement. This in turn supports its use in immersion cooling applications which is shown for the first time.
LINK to Repository: https://www.researchgate.net
ABSTRACT
An efficient thermal management system is crucial for the best performance of heavy-duty electric vehicle (EV) battery packs. The requirement of high heat removal necessitates the use of liquid coolants leading to higher heat transfer coefficient as compared to the conventional air-cooled systems. However, higher viscosity of the liquid coolants increases the pumping power and reduces mixing in the thermal boundary layer (TBL), hence reducing the coefficient of performance (COP, ratio of the heat transfer rate to the pumping power) of the cooling system. We demonstrate a novel immersion cooling strategy, which uses shear-thinning viscoelastic fluids with millimetric structures on the heat transfer surface. The shear-thinning property reduces the pumping power, while the viscoelastic properties generate elastic instabilities on carefully designed surface structures and thereby promote mixing in the TBL. We optimize the shape of the surface structures and properties of the cooling liquids to achieve maximum COP using computational fluid dynamics (CFD) simulation in OpenFOAM. The viscoelastic model liquids are made of solvents and polymers with known relaxation times. We explore the parameter range including solvent and polymer viscosities, relaxation time of the polymers and shape of the surface structures, which provide the initial design guidelines for an experimental setup. Our method is expected to enhance the performance of heavy-duty battery thermal management systems as compared to that of the state-of-the-art solutions.
LINK to Repository: https://www.researchgate.net
ABSTRACT
In many novel applications, batteries undergo high charging and discharging rates, which, among other effects, leads to high thermal wear. Specifically the lifetime of Li-ion batteries reduces significantly under operation outside of a certain range of operating temperatures, which severely impairs the sustainability of the related applications. As air-cooling and pipe-based cooling systems often do not provide enough cooling power or temperature homogeneity, there currently exits strong interest in immersed battery cooling systems. To reach optimum heat transfer at minimum pumping power, the topography of the surface exposed to the coolant, as well as the coolant properties have to be optimized. Especially shear-thinning viscoelastic liquids seem to be promising candidates for coolants, as secondary flow patterns can arise in their flow at low Reynolds numbers. We here present an experimental setup, which allows evaluating different combinations of surface topography and coolant in a flow cavity with a geometry, which is directly relatable to flow between neighboring battery cells. We measure the maximum temperature, as well as the temperature gradient in flow direction of a heated structured sample plate exposed to flow of either pure water or water with 0.1% (w/w) xanthan added. We find that surface topography, which leads to a better cooling performance in combination with the former (Newtonian) coolant, loses this advantage when used with the latter (shear-thinning viscoelastic) coolant. We also find indications for secondary flow arising in the non-Newtonian liquid in combination with another surface geometry. This will help guiding the design of surface topographies for shear-thinning viscoelastic coolants.
LINK to Repository: https://doi.org/10.1021/acs.iecr.4c01832
ABSTRACT
A comparative assessment of the thermal properties and heat transfer coefficients achieved by viscoelastic nanofluids suitable for immersion cooling is presented, with the candidate samples exhibiting distinct differences based on the nanoparticle chemistry and shape. Molecular dynamics simulations of different nanoparticles such as copper nanosphere, two-dimensional pristine graphene, and single-walled carbon nanotube (CNT) dispersed in PAO-2 of concentrations of approximately equal to 2.6% by weight are performed in the present investigation. While carbon-based nanoparticles increase the specific heat capacity of the nanofluids, copper-based nanofluids show a decrease in the corresponding values. Moreover, the heat conduction in copper-based nanofluids is dependent on the higher degree of phonon density of states (DOS) matching between the copper and solvent atoms, whereas the high intrinsic thermal conductivity of graphene and CNT compensates for the lower degree of DOS matching. The addition of an OCP polymer chain to impart viscoelasticity in the nanofluids exhibits a heat transfer coefficient enhancement of more than 80% during Couette flow as a result of chain expansion, indicating their suitability for immersive-cooling applications.
LINK to Repository: https://doi.org/10.1016/j.polymer.2023.126360
ABSTRACT
A framework predicting the rheological (storage and loss moduli, first normal stress coefficient, and relaxation time) and transport (viscosity, diffusivity) properties of non-Newtonian dilute polymer solutions at mesoscales (e.g. ∼ns to μs) from the atomistic-scale molecular behaviour is presented. More specifically, the rheological behaviour differences of OCP and PMA polymer solutions in PAO-2 oil are simulated using both atomistic molecular dynamics (MD) and many-body dissipative particle dynamics (mDPD) within a temperature range of 313–373 K. The simulation methodology described is able to distinguish itself from the standard DPD model by accurately reproducing the shear-thinning with high sensitivity, as seen in the atomistic MD simulations at high shear rates (e.g. 108−1013 s−1). It is shown that the model is well-suited to compute properties such as first normal stress differences and relaxation times that are difficult to estimate at atomistic scales due to the low signal-to-noise ratio. Moreover, the Schmidt numbers (>103) are predicted with high accuracy when compared with the values from atomistic-scale simulations. The proposed model is able to predict relaxation times of dilute polymer mixtures that are difficult to be obtained using state-of-the-art rheometers. Finally, it is found that the terminal relaxation time of PMA polymer chain does not vary monotonically as a function of temperature, unlike in the case of OCP; this is significant for describing viscoelastic behaviour at macroscales where satisfactory constitutive equations are not available.
LINK to Repository: https://doi.org/10.1038/s41598-024-76588-3
ABSTRACT
The current experimental investigation demonstrates the capability of neutron imaging to quantify cavitation, in terms of vapour content, within an orifice of an abruptly constricting geometry. The morphology of different cavitation regimes setting in was properly visualised owing to the high spatial resolution of 16 μm achieved, given the extensive field of view of 12.9 × 12.9 mm2 offered by the imaging set-up. At a second step, the method was proven capable of highlighting subtle differences between fluids of different rheological properties. More specifically, a reference liquid was comparatively assessed against a counterpart additised with a Quaternary Ammonium Salt (QAS) agent, thus obtaining a viscoelastic behaviour. In accordance with previous studies, it was verified, yet in a quantifiable manner, that the presence of viscoelastic additives affects the overall cavitation topology by promoting the formation of more localised vortical cavities rather than cloud-like structures occupying a larger portion of the orifice core. To the authors’ best knowledge, the present work is the first to demonstrate that neutron imaging is suitable for quantifying in-nozzle cavitating flow at the micrometre level, consequently elucidating the distinct forms of vaporous structures that arise. The potential of incorporating neutron irradiation for the quantification of two-phase flows in metallic microfluidics devices has been established.
LINK to Repository: https://www.researchgate.net
ABSTRACT
Battery thermal management systems (BTMSs) for electric vehicles are of primary importance to the efforts for further market penetration of electric vehicle (EV) technology, as thermal safety and performance issues of Li-ion batteries persist. Different battery cooling approaches have been developed to address the issues of maximum temperature and temperature uniformity. In this paper, a novel, immersion-cooling BTMS concept is proposed where oil-based, dielectric fluids are employed. The cooling performance is further improved by incorporating linear vortex generators (LVGs) to enhance mixing close to the battery cell consequently heat transfer. The proposed BTMS is tested under three different rates of discharge (1C, 5C, 10C) with air and dielectric-fluid cooling. The results indicate that the use of a dielectric fluid along with LVGs leads to a significant drop of the maximum battery cell temperature and a temperature uniformity of 1 o C across the battery cell. INTRODUCTION One of the most promising technological developments to reduce global carbon footprint in transportation is power train electrification, with lithium-ion (Li-ion) batteries being widely used in electric vehicles due to their high energy-density and power-energy attributes. These attractive attributes however naturally lead to increased heat generation rates affecting the thermal performance and safety of the battery packs. Temperature control and management of the Li-ion battery packs are vital for long-term performance, life cycle and durability. Cell performance varies significantly with temperature, while the electrochemical phenomena occurring within the battery cells are also temperature dependent. The suggested window of operation for the temperature of a Li-ion battery is between 15 o C-60 o C, with the optimum temperature being below 40 o C. Temperature uniformity within each battery cell and among different cells of the same battery pack is also of great importance, with a maximum of 5 o C being the target. Battery thermal management systems (BTMS) play an important role in maintaining temperature levels within the desired window and promoting temperature uniformity, thus promoting battery life cycle and preventing issues like thermal runaway and battery degradation.
LINK to Repository: https://www.researchgate.net
ABSTRACT
The effectiveness of immersion cooling for the thermal managementof Electric-Vehicle (EV) batteries is crucially influenced by thethermophysical and rheological properties of the heat-transfer liquid.This study emphasizes upon the design requirements for such a fluidin terms of bulk properties, i.e., high electrical resistivity and thermalconductivity, low viscosity, but also relevant to the rheologicalproperties maximizing the heat transfer rate. Key concepts of theimplemented research constitute: (i) the promotion of vortical motionin the laminar flow regime, which, in turn, enhances heat transfer bydisrupting boundary layers; (ii) vortex stabilization through theaddition of viscoelasticity-inducing agents in the base heat-transferliquid. To improve cooling efficiency, the primary objective is tomaximize the achievable heat transfer rate for minimal pumpinglosses. Hence, a multi-objective optimization process must be set inplace where the optimal coolant rheology is dependent on thegeometrical features of the battery module. The overall framework ofinterdependent research activities comprises: (i) the characterizationof viscoelastic flow with the use of Particle Image Velocimetry (PIV)in a flow loop with benchmark geometries; (ii) heat-transfermeasurements employing a novel Atom Layer Thermopile (ALTP)sensor and (iii) dedicated computational fluid dynamics (CFD)modelling using the Phan-Thien-Tanner constitutive equation forelastic stresses. While there are tailored designs for efficient heattransfer in immersion-cooling paradigms needed, in this paper weconcentrate on heat flux measurements when cooling a bluff body. Inthis work results on heat transfer in the wake behind a square rodwere analyzed and discussed. High-viscosity liquids have higher heattransfer at equivalent Re-number. An improvement of heat-transferdue to viscoelastic flow behavior is indicated for high-viscosityliquids, but the trend must be proven with additional experiments.PIV based flow analysis shows a mismatch between the flow patternand the heat transfer surface.
LINK to Repository: https://www.researchgate.net
ABSTRACT
Thermal management of electric high-power devices is more important than ever. The electrification of the transport sector and increasing High-Performance-Computing (HPC) are the main driving forces for the increasing demands. Therefore, immersion cooling concepts arise, where the cooling liquid is in direct contact with the heat source, e.g., the CPU of a computer or the cell of a battery module. The main advantage is to eliminate the heat conduction through a cooling plate or heat pipes in the heat transfer path, which increases cooling efficiency. The downside is, that the cooling system could get very bulky, which is not very critical in huge HPC centers but is a showstopper for mobile applications. The project IBAT investigates radically new dielectric cooling fluids, funded by the EU program horizon 2020. These oil-based synthetic liquids are of very low viscosity. This allows very small gaps between battery cells resulting in a compact battery module design. Naturally appearing vortices at the cell edges quickly dissolve due to the low viscosity liquids. A stable boundary layer forms on the cell surface diminishing the heat transfer. To counteract this phenomenon linear vortex generators (LVGs) could be applied to the cell surface. The downside of this solution is increased flow resistance and thus higher pumping losses in the cooling circuit, partly canceling out the advantages of the low viscosity liquids. The more sophisticated solution to break up the thermal boundary layer is to stabilize the natural vortices. This is achieved by adding long polymer chains to the fluids, which give them viscoelastic properties. The initial results are very promising but show that the fluid properties must be tailored to the specific geometry. The catch is that the viscoelastic fluid properties of such low-viscosity fluids cannot be measured with conventional rheometers. A workaround is provided by optical flow analytics in benchmark geometries. One method is the observation of cross-plane vortices in a 180° bend with circular cross-section. The authors describe and discuss the challenges and uncertainties of cross-plane PIV for such experiments.
LINK to Repository: https://pdf.sciencedirectassets.com/
ABSTRACT
Effective thermal management plays a critical role in ensuring peak performance for heavy-duty electric vehicle battery packs and high-performance electronics. To address the need for effective heat dissipation, liquid coolants are employed, which exhibit higher heat transfer performance compared to traditional air-cooled systems. However, the associated increased viscosity of such liquid coolants and constricted flow path lead to reduced mixing, thereby deteriorating the thermal performance of the system. Here, we explore the potential of utilizing viscoelastic liquid coolants for enhancing heat transfer from the heated walls of a constricted flow path with obstacles, modeling features of battery coolant channels. At low-Reynolds number regime, viscoelastic fluids exhibit instabilities which can disrupt the thermal boundary layer and enhance fluid mixing in the domain, thereby facilitating wall heat transfer. We employed computational fluid dynamics simulations, validated with experimental observations, to explore the effect of the flow geometry, Weissenberg number, solvent viscosity ratio, polymer extensibility, and Reynolds number on thermal performance. Our study indicates that the heat transfer is enhanced with flow instabilities in viscoelastic fluids, which can be generated exclusively in the flow channel with obstacles by increasing the Weissenberg number, reducing the solvent viscosity ratio, increasing the polymer extensibility, and increasing the Reynolds number. The enhancement in the heat transfer can exclusively be attributed to the elastic properties of the fluid in contrast to the inelastic shear-thinning fluids exhibiting no instability. The insights from the study can play an important role in the development of advanced, high-efficiency battery cooling systems, combining surface engineering and synthesis of optimized heat transfer liquids.