Search

Experiments conducted by NASA measured the pressure drop due to gas-liquid flow through a packed-bed reactor under microgravity conditions. From these experiments, we develop correlations for the gas-liquid $f_{gl}$ interphase drag in a two-fluid model (TFM). We use an Ergun-type closure for liquid-solid drag. Then, under a 1D flow assumption, $f_{gl}$ is the only unknown in the TFM. Using a data-driven approach, we determine $f_{gl}$ and correlate it (via composite fits) with the liquid and gas Reynolds numbers, $Re_{l}$ and $Re_{g}$, respectively, and the Suratman number $Su_{l}$. To validate the proposed $f_{gl}(Re_{l},Re_{g},Su_{l})$ closure, we perform two-dimensional transient simulations at microgravity conditions using ANSYS Fluent and employing an Euler-Euler formulation. We find good agreement between the simulations based on the proposed $f_{gl}$ closure and the experimental data., Comment: 25 pages, 5 figures, preprint mode; v2: minor updates/enhancements

Battery thermal management systems (BTMSs) are critical for efficient and safe operation of lithium-ion batteries (LIBs), especially for fast charging/discharging applications that generate significant heating within the cell. Forced immersion cooling, where a dielectric fluid flows in direct contact with the LIB cells, is an effective cooling approach. But because of its complex nature, a thorough understanding of the underlying physics - including the coupled electrochemical, thermal, fluid, and mechanical effects - is required before immersion cooling will see wide adoption into commercial systems. In this work, to investigate the performance of a LIB subjected to forced immersion cooling, we develop a fully coupled modeling approach that solves the detailed electrochemical model in conjunction with the thermal-fluid transport models for both the cell and fluid domain. After calculating the electrochemical and thermal responses, we also estimate the mechanical stresses within the cell generated due to the ion diffusion and temperature rise that impact reliability. To assess the effectiveness of forced immersion cooling, we evaluate several different configurations for a cylindrical 18650 battery cell under varying cell discharge rates. We compare forced immersion cooling for two liquids (deionized water and mineral oil) at three different fluid mass flow rates. The results highlight the strong cross-coupling of the electrochemical and heat transfer phenomena. By comparing results across fluids and flow rates, we define a new metric that can be used to compare the cooling capacity considering different flow parameters. Overall, this study provides insights that will be useful in the design of immersion cooling-based BTMSs including, for example, the selection of forced immersion cooling specifications, such that the temperature is controlled without significant capacity loss., Comment: 39 pages; 11 figures

Gas-liquid flows through packed bed reactors (PBRs) are challenging to predict due to the tortuous flow paths that fluid interfaces must traverse. Experiments at the International Space Station showed that bubble and pulse flows are predominately observed under microgravity conditions, while the trickle and spray flows observed under terrestrial conditions are not present in microgravity. To understand the physics behind the former experiments, we simulate bubble flow through a PBR for different packing-particle-diameter-based Weber numbers and under different gravity conditions. We demonstrate different pore-scale mechanisms, such as capillary entrapment, buoyancy entrapment, and inertia-induced bubble displacement. Then, we perform a quantitative analysis by introducing new dynamic scales, dependent upon the evolving gas-liquid interfacial area, to understand the dynamic trade-offs between the inertia, capillary, and buoyancy forces on a bubble passing through a PBR. This analysis leads us to define new dimensionless Weber-like numbers that delineate bubble entrapment from bubble displacement., Comment: 12 pages, 9 figures; v2: minor updates, to appear in the AIChE Journal

The lattice thermal conductivity ($\kappa$) of two ceramic materials, cerium dioxide (CeO$_2$) and magnesium oxide (MgO), is computed up to 1500 K using first principles and the phonon Boltzmann Transport Equation (PBTE) and compared to time-domain thermoreflectance (TDTR) measurements up to 800 K. Phonon renormalization and the four-phonon effect, along with high temperature thermal expansion, are integrated in our \textit{ab initio} molecular dynamics (AIMD) calculations. This is done by first relaxing structures and then fitting to a set of effective force constants employed in a temperature-dependent effective potential (TDEP) method. Both three-phonon and four-phonon scattering rates are computed based on these effective force constants. Our calculated thermal conductivities from the PBTE solver agree well with literature and our TDTR measurements. Other predicted thermal properties including thermal expansion, frequency shift, and phonon linewidth also compare well with available experimental data. Our results show that high temperature softens phonon frequency and reduces four-phonon scattering strength in both ceramics. Compared to MgO, we find that CeO$_2$ has weaker four-phonon effect and renormalization greatly reduces its four-phonon scattering rates.

Thermal greases, often used as thermal interface materials, are complex paste-like mixtures composed of a base polymer in which dense metallic (or ceramic) filler particles are dispersed to improve the heat transfer properties of the material. They have complex rheological properties that impact the performance of the thermal interface material over its lifetime. We perform rheological experiments on thermal greases and observe both stress relaxation and stress buildup regimes. This time-dependent rheological behavior of such complex fluid-like materials is not captured by steady shear-thinning models often used to describe these materials. We find that thixo-elasto-visco-plastic (TEVP) and nonlinear-elasto-visco-plastic (NEVP) constitutive models characterize the observed stress relaxation and buildup regimes respectively. Specifically, we use the models within a data-driven approach based on physics-informed neural networks (PINNs). PINNs are used to solve the inverse problem of determining the rheological model parameters from the dynamic response in experiments. This training data is generated by startup flow experiments at different (constant) shear rates using a shear rheometer. We validate the ``learned'' models by comparing their predicted shear stress evolution to experiments under shear rates not used in the training datasets. We further validate the learned TEVP model by solving a forward problem numerically to determine the shear stress evolution for an input step-strain profile. Meanwhile, the NEVP model is further validated by comparison to a steady Herschel--Bulkley fit of the material's flow curve., Comment: 13 pages, 10 figures; v2 included new validation appendices; to appear in J. Rheol

We study 4d Friedmann-Lema\^{i}tre-Robertson-Walker cosmologies obtained from time-dependent compactifications of Type IIA 10d supergravity on various classes of 6d manifolds (Calabi-Yau, Einstein, Einstein-K\"{a}hler). The cosmologies we present are universal in that they do not depend on the detailed features of the compactification manifold, but only on the properties which are common to all the manifolds belonging to that class. Once the equations of motion are rewritten as an appropriate dynamical system, the existence of solutions featuring a phase of accelerated expansion is made manifest. The fixed points of this dynamical system, as well as the trajectories on the boundary of the phase space, correspond to analytic solutions which we determine explicitly. Furthermore, some of the resulting cosmologies exhibit eternal or semi-eternal acceleration, whereas others allow for a parametric control on the number of e-foldings. At future infinity, one can achieve both large volume and weak string coupling. Moreover, we find several smooth accelerating cosmologies without Big Bang singularities: the universe is contracting in the cosmological past ($T<0$), expanding in the future ($T>0$), while in the vicinity of $T=0$ it becomes de Sitter in hyperbolic slicing. We also obtain several cosmologies featuring an infinite number of cycles of alternating periods of accelerated and decelerated expansions., Comment: v1: 38 pages + appendices, 18 figures; v2-v4: minor modifications and corrected typos

Classical flux compactifications contribute to a well-controlled corner of the string landscape, therefore providing an important testing ground for a variety of conjectures. We focus here on type II supergravity compactifications on 6d group manifolds towards 4d maximally symmetric spacetimes. We develop a code where the truncation to left-invariant scalars and the dimensional reduction to a 4d theory are automated, for any possible configuration of Op-planes and Dp-branes. We then prove that any such truncation is consistent. We further compute the mass spectrum and analyse the stability of many de Sitter, Minkowski or anti-de Sitter solutions, as well as their consistency with swampland conjectures., Comment: 31 pages. The numerical code MSSV and a corresponding database are provided as ancillary files; v2: few additions; v3: improvement of the scalar potential expressions in the paper, and of the analysis at a generic field point in the code

We classified in arXiv:2201.04152 certain 10d supergravity solutions with a 4d de Sitter, Minkowski or anti-de Sitter spacetime. We then found new solutions in previously unexplored classes. In this paper we study their properties, compare them to swampland conjectures, and make new observations. Using new numerical tools, we first identify all Lie algebras underlying the 6d group manifolds, allowing us to discuss their compactness. We then investigate scale separation, and prove related no-go theorems. Last but not least, we automatize and analyze the stability of all solutions. This leads us to propose the Massless Minkowski Conjecture, claiming the systematic presence of a 4d massless scalar field., Comment: v1: 30 pages + appendices. The numerical codes AlgId, AlgIso and MaxSymSolSpec developed are provided here as ancillary files; v2: minor modifications, published version; v3: clarification on the Massless Minkowski Conjecture

We classify solutions of 10d type IIA/B supergravities with orientifolds, on a 4d maximally symmetric spacetime times a 6d group manifold. We then look for new solutions in previously unexplored solution classes, and find some: (anti-) de Sitter solutions with intersecting O4, O6 and D6, or Minkowski solutions with 3 intersecting O5, among others. We provide the numerical code that we developed for this purpose. We also prove new no-go theorems against (anti-) de Sitter solutions. We finally conjecture the absence of de Sitter solution for 2 or less intersecting source sets, implying that a 4d effective theory with de Sitter is at most N=1 supersymmetric., Comment: v1: 33 pages + appendices. The numerical code MaxSymSolSearch developed is provided here as an ancillary file. Two further Mathematica notebooks listing the old and new solutions are also provided. v2: minor additions. v3: few additions, published version

Metal hydrides have been studied for use in energy storage, hydrogen storage, and air-conditioning (A/C) systems. A common architecture for A/C and energy storage systems is two metal hydride reactors connected to each other so that hydrogen can flow between them, allowing for cyclic use of the hydrogen. This paper presents a nonlinear dynamic model and multivariate control strategy of such a system. Each reactor is modelled as a shell-and-tube heat exchanger connected to a circulating fluid, and a compressor drives hydrogen flow between the reactors. We further develop a linear state-space version of this model integrated with a model predictive controller to determine the fluid mass flow rates and compressor pressure difference required to achieve desired heat transfer rates between the metal hydride and the fluid. A series of case studies demonstrates that this controller can track desired heat transfer rates in each reactor, even in the presence of time-varying circulating fluid inlet temperatures, thereby enabling the use of a two-reactor system for energy storage or integration with a heat pump., Comment: 42 pages; 14 Figures

We develop a two-fluid model (TFM) for heat transfer in dense non-Brownian suspensions. Specifically, we propose closure relations for the inter-phase heat transfer coefficient and the thermal diffusivity of the particle phase based on calibration against experimental data. The model is then employed to simulate non-isothermal flow in an annular Couette cell. We find that, when the shear rate is controlled by the rotation of the inner cylinder, both the shear and thermal gradients are responsible for particle migration. Within the TFM framework, we identify the origin and functional form of a "thermo-rheological" migration force that rationalizes our observations. Furthermore, we apply our model to flow in eccentric Couette cells. Our simulations reveal that the system's heat transfer coefficient is affected by both the classic shear-induced migration of particles and the newly identified thermo-rheological migration effect., Comment: 20 pages, 11 figures; v2: minor revisions, accepted for publication in International Journal of Heat and Mass Transfer

Leveraging the latent heat of phase change materials (PCMs) can reduce the peak temperatures and transient variations in temperature in electronic devices. But as the power levels increase, the thermal conduction pathway from the heat source to the heat sink limits the effectiveness of these systems. In this work, we evaluate embedding the PCM within the silicon device layer of an electronic device to minimize the thermal resistance between the source and the PCM to minimize this thermal resistance and enhance the thermal performance of the device. The geometry and material properties of the embedded PCM regions are optimized using a combination of parametric and machine learning algorithms. For a fixed geometry, considering commercially available materials, Solder 174 significantly outperforms other organic and metallic PCMs. Also with a fixed geometry, the optimal melting points to minimize the peak temperature is higher than the optimal melting point to minimize the amplitude of the transient temperature oscillation, and both optima increase with increasing heater power. Extending beyond conventional optimization strategies, genetic algorithms and particle swarm optimization with and without neural network surrogate models are used to enable optimization of many geometric and material properties. For the test case evaluated, the optimized geometries and properties are similar between all ML-assisted algorithms, but the computational time depends on the technique. Ultimately, the optimized design with embedded phase change materials reduces the maximum temperature rise by 19% and the fluctuations by up to 88% compared to devices without PCM., Comment: 13 pages, 8 figures, 7 tables

A distinct signature of compact extra dimensions would be a Kaluza-Klein tower of gravitational waves. Motivated by this prospect, we compute the corresponding spectrum on a warped toroidal background. We evaluate in particular the impact of the warp factor on the spectrum. To that end, we use the complete warp factor H of standard string compactifications, generated by D-branes and orientifolds, thus connecting to recent works on stringy de Sitter constructions. The problematic region close to an orientifold where H < 0 leads to unphysical tachyonic modes in the spectrum. We develop tools that overcome this difficulty and lead to a tachyon-free spectrum. We show, in particular, that the warp factor can lower the first Kaluza-Klein mass by at least 69%., Comment: v2: 38 pages, 11 figures, minor modifications and few additions, published version

Up-to-date, there is no known example of a classical de Sitter solution of string theory, despite several good candidates. We consider here two newly discovered 10d supergravity de Sitter solutions, and analyse in great detail whether they can be promoted to classical string backgrounds. To that end, we identify five requirements to be met, and develop the necessary 10d tools to test the solutions. Eventually, they both fail to verify simultaneously all requirements, in spite of positive partial results. The explicit values obtained offer a clear illustration of the situation, and the analysis highlights various subtleties. We finally discuss the relation to the problem of scale separation., Comment: 9 pages; v2: few additions on scale separation; v3: small additions, published version

We find and study 17 new de Sitter solutions of ten-dimensional (10d) type IIB supergravity with intersecting D5-branes and orientifold O5-planes, as well as a new Minkowski one. These solutions are obtained numerically on 6d group manifolds, the compactness of which is established for 4 of them. We show that all our de Sitter solutions are perturbatively unstable, using a restricted 4d effective theory of four scalar fields. We finally analyse whether our solutions can be promoted to classical string backgrounds. Several of them appear as good candidates, as they satisfy all requirements imposed so far., Comment: 28 pages + appendices; v2: minor additions; v3: minor changes, published version