Browsing by Author "Laitinen, Alpo"

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Computational fluid dynamics modeling of heat transfer in a LuxTurrim5G smart light pole
(2021-08-23) Kulkarni, Pranav
Insinööritieteiden korkeakoulu | Master's thesis
The present thesis is a part of the LuxTurrim5G project, a collaborative initiative of Business Finland and participating companies. The objective of the thesis is to simulate turbulent flow inside of a utility box with a flow rate of \SI{2}{m^3/min} to analyze the thermal behaviour of the system. The utility box houses several electronic components some of which produce heat and therefore need cooling for optimum performance. Two different flow directions were considered to evaluate the effectiveness of each of them. Four different temperature fields were used to account for the different ambient temperature conditions and solar radiation on the outer wall in two different locations on Earth, Helsinki and the Equator (in Africa). The analysis was carried out using the RANS (Reynolds Averaged Navier Stokes) simulations and LES (Large Eddy Simulations) was used to check for the turbulent effects in the domain. The time-averaged fields in the LES cases were found to be very similar to the flow fields in the RANS simulations. The only major difference was the maximum temperature being slightly lower in the LES cases owing to the transition effects from RANS to LES. The CFD (Computational Fluid Dynamics) simulations were carried out on the Finnish supercomputer and were run until a steady state was reached. The complex geometry was simplified to ease the meshing process to produce a fairly consistent mesh. Four different mesh sizes were used to prove grid independence and the large eddy simulations were initialized with the steady state RANS solutions. Several flow features like flow recirculation were observed because of the rectangular blocks in the domain and these regions were regions of high heat concentrations. The maximum temperature reached was at the Equatorial conditions due to high ambient temperatures and intense solar radiation. The highest local air temperatures in the domain were close to \SI{90}{^oC}. However, simplification of the geometry lead to the removal of heat removal locations for the heated blocks and also, some passive cooling effects were neglected. It was concluded, based on the temperature fields and the probability distribution function of temperature that the top-to-bottom flow removes heat better from the system all of the different surface temperatures used and therefore, the surface temperatures do not have a major impact on the heat removal rate of the system.
Computational fluid dynamics simulations of liquid cooling for electronics
(2018-10-29) Laitinen, Alpo
Insinööritieteiden korkeakoulu | Master's thesis
This work was done as part of the LuxTurrim5G project funded by the participating companies and Business Finland. The objective was to design a compact liquid cooling heat sink for electronics. Two different flow geometries for an aluminum heat sink for liquid cooling were designed and numerically analyzed with computational fluid dynamics (CFD) using a conjugate heat transfer (CHT) model. In one geometry, guiding vanes at the inlet were used to decrease maldistribution of the fluid in the heatsink, and an empty, large opening in the other. Both geometries were studied with two different flow rates of the cooling liquid, corresponding to inlet Reynolds numbers of 4171 and 15641. For the modeling of turbulence, the standard k-epsilon model was used. Water was used as the heat transfer fluid in all simulations. The heat applied to the heatsink was set to 1600W, which is considered a worst case scenario. The highest allowed surface temperature to assure proper functionality of the electronics was set to 338K. The inlet water temperature was set to 300K. With 2 L/min flow rate, the surface temperature was noted to reach levels of 346.7K without the guiding vanes and 341.9K with the guiding vanes. With 7 L/min flow rate the surface temperatures were noted to reach levels of 332.7 without the guiding vanes and 326.4 with the guiding vanes. The effect of the guiding vanes and the flow rate of the cooling liquid were observed to be significant. In addition, the heat transfer enhancement in the entrance region of a developing laminar flow was observed to be of great importance. The reliability of the results obtained with the k-epsilon model was assessed by comparing the results to large-eddy simulation (LES). The comparison revealed similarities in the flow field within decent accuracy. The CHT solver used in this thesis was also validated against an analytic solution of a laminar flow between heated infinite parallel plates. The numeric results were observed to be in good agreement with the analytic solutions.
Computational fluid dynamics simulations of thermal flows in various applications
(2023) Laitinen, Alpo
School of Engineering | Doctoral dissertation (article-based)
The present thesis belongs to the field of computational physics. In particular, computational fluid dynamics (CFD) methods are utilized to simulate heat transfer and fluid flow phenomena, with the methodological emphasis on using scale-resolving large-eddy simulations (LES). The overall objective of the thesis is to utilize LES in various societally relevant applications on various scales. The dissertation is among pioneering works on the usage of LES in conjugate heat transfer, recovery boiler, and indoor airflow context. In Publication I, a liquid cooling heat exchanger with embedded channels for high power electronics was designed and numerically analyzed. Experiments carried by co-authors are also reported in the publication to confirm the functionality of the heat exchanger. The numerical analysis was performed with three different turbulence models, and the comparison revealed significant differences in the thermal performance based on the capturing of the relevant flow features. In particular, the overprediction in the level of turbulence was observed to also overpredict the turbulent heat transfer and consequently, underpredict the surface temperature levels. In Publication II, a kraft recovery boiler with two different secondary air jet configurations was analyzed with LES. In addition to the flow field, dispersion of black liquor droplets was simulated as solid particles with Lagrangian particle tracking (LPT). The two jet configurations exhibited differences in the mixing of the hot and cool gases as well as in the dispersion of the particles. A staggered jet configuration provided more uniform temperature levels compared to an in-line configuration. However, significantly more fouling was observed in the staggered configuration. In Publication III, a large room with mechanical ventilation was simulated using LES. The simulation case was motivated by a real life SARS-CoV-2 virus super-spreading event during a choir practice. The emphasis of the simulation was on the buoyancy-driven flows generated by the radiators and the choir attendees, and the effect of buoyancy on the dispersion of the respiratory aerosols. The presence of buoyancy was noted to increase the turbulent dispersion of the respiratory aerosols significantly. Pope's criterion was utilized to verify the numerical solution in Publication II and III. As an overall conclusion, usage of scale-resolving techniques (e.g. LES) appears important in predicting flows with transitional features and strong turbulence generation affecting e.g particle dispersion patterns or wall heat transfer.
Efficient storage and recovery of waste heat by phase change material embedded within additively manufactured grid heat exchangers
(2021-12) Yazdani, Maryam Roza; Laitinen, Alpo; Helaakoski, Valtteri; Farnas, Lorant Katona; Kukko, Kirsi; Saari, Kari; Vuorinen, Ville
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The low thermal conductivity of organic phase change materials (PCMs) hinders their usage for energy storage purposes. We demonstrate a compact PCM-based thermal battery that employs three-dimensional (3D) printed metal surfaces for a robust thermal energy storage and recovery. The thermal battery could be utilized to store excess heat from various sources. The concept includes organic paraffin and fatty acid PCMs embedded within aluminum silicon alloy grid heat exchangers (GHE) produced via additive manufacturing. The heat exchangers consist of two parts: (i) a planar part with embedded water channels and (ii) a surface extrusion grid outside the planar part embedded in the PCM storage system. Three different grid designs are investigated and compared with a simple planar heat exchanger (PHE) without grid extension. The charging and discharging processes of the thermal battery were analyzed experimentally. The laboratory scale experiments reveal that the 3D printed grid surfaces of GHE significantly reduce the charging and discharging time from more than 240 min to less than 20 min. In contrast to PHE, the GHE may increase thermal power by a factor of ∼20 from 35 W to 670 W. Furthermore, the grid structure positively restrains the natural convection flow of the PCM melt, increasing conduction in the highly conductive grid structure and resulting in high charging-discharging power of the thermal battery. The swift charging and discharging with high power and energy density make the compact grid thermal battery a promising solution for thermal energy management.
A GPU-accelerated computational fluid dynamics solver for assessing shear-driven indoor airflow and virus transmission by scale-resolved simulations
(2024-06) Korhonen, Marko; Laitinen, Alpo; Isitman, Gizem Ersavas; Jimenez, Jose L.; Vuorinen, Ville
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We explore the applicability of MATLAB for 3D computational fluid dynamics (CFD) of shear-driven indoor airflows. A new scale-resolving, large-eddy simulation (LES) solver titled DNSLABIB is proposed for MATLAB utilizing graphics processing units (GPUs). In DNSLABIB, the finite difference method is applied for the convection and diffusion terms while a Poisson equation solver based on the fast Fourier transform (FFT) is employed for the pressure. The immersed boundary method (IBM) for Cartesian grids is proposed to model solid walls and objects, doorways, and air ducts by binary masking of the solid/fluid domains. The solver is validated in two canonical reference cases and against experimental data. Then, we demonstrate the validity of DNSLABIB in a room geometry by comparing the results against another CFD software (OpenFOAM). Next, we demonstrate the solver performance in several isothermal indoor ventilation configurations and the implications of the results are discussed in the context of airborne transmission of COVID-19. The novel numerical findings using the new CFD solver are as follows. First, a linear scaling of DNSLABIB is demonstrated and a speed-up by a factor of 3-4 is also noted in comparison to similar OpenFOAM simulations. Second, ventilation in three different indoor geometries are studied at both low (0.1 m/s) and high (1 m/s) airflow rates corresponding to Re=5000 and Re=50000. An analysis of the indoor CO2 concentration is carried out as the room is emptied from stale, high CO2 content air. We estimate the air changes per hour (ACH) values for three different room geometries and show that the numerical estimates from 3D CFD simulations differ by 80%–150% (Re=50000) and 75%–140% (Re=5000) from the theoretical ACH value based on the perfect mixing assumption. Third, the analysis of the CO2 probability distributions (PDFs) indicates a relatively non-uniform distribution of fresh air indoors. Fourth, utilizing a time-dependent Wells-Riley analysis, an example is provided on the growth of the cumulative infection risk which is shown to reduce rapidly after the ventilation is started. The average infection risk is shown to reduce by a factor of 2 for lower ventilation rates (ACH=3.4-6.3) and 10 for the higher ventilation rates (ACH=37-64). Finally, we utilize the new solver to comment on respiratory particle transport indoors. A key contribution of the paper is to provide an efficient, GPU compatible CFD solver environment enabling scale-resolved simulations (LES/DNS) of airflow in large indoor geometries on a single GPU designed for high-performance computing. The demonstrated efficacy of MATLAB for GPU computing indicates a high potential of DNSLABIB for various future developments on airflow prediction.
Lämmönjohtavuuden mekanismit nanonesteissä
(2016-04-18) Laitinen, Alpo
Insinööritieteiden korkeakoulu | Bachelor's thesis
Large-eddy simulation of buoyant airflow in an airborne pathogen transmission scenario
(2023-08-01) Laitinen, Alpo; Korhonen, Marko; Keskinen, Karri; Kaario, Ossi; Vuorinen, Ville
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Indoor airflow patterns and the spreading of respiratory air were studied using the large-eddy simulation (LES) computational fluid dynamics (CFD) approach. A large model room with mixing ventilation was investigated. The model setup was motivated by super-spreading of the SARS-CoV-2 virus with a particular focus on a known choir practice setup where one singer infected all the other choir members. The room was heated with radiators at two opposite walls in the cold winter time. The singers produced further heat generating buoyancy in the room. The Reynolds number of the inflow air jets was set to Re=2750, corresponding to an air-changes-per-hour (ACH) value of approximately 3.5. The CFD solver was first validated after which a thorough grid convergence study was performed for the full numerical model room with heat sources. The simulations were then executed over a time of t=20 min to account for slightly more than one air change timescale for three model cases: (1) full setup with heat sources (radiators+singers) in the winter scenario, (2) setup without radiators in a summer scenario, and (3) theoretical setup without buoyancy (uniform temperature). The main findings of the paper are as follows. First, the buoyant flow structures were noted to be significant. This was observed by comparing cases 1/2 with case 3. Second, the dispersion of the respiratory aerosol concentration, modeled as a passive scalar, was noted to be significantly affected by the buoyant flow structures in cases 1–2. In particular, the aerosol cloud was noted to either span the whole room (cases 1–2) or accumulate in the vicinity of the infected singer (case 3). Turbulence was clearly promoted by the interaction of the upward/downward moving warmer/cooler air currents which significantly affected the dispersion of the respiratory aerosols in the room. The study highlights the benefits of high-resolution, unsteady airflow modeling (e.g. LES) for interior design which may consequently also impact predictions on exposure to potentially infectious respiratory aerosols.
Large-Eddy Simulation of two secondary air supply strategies in kraft recovery boilers
(2022-11-05) Laitinen, Alpo; Laurila, Erkki; Keskinen, Karri; Vuorinen, Ville
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Kraft recovery boilers are large scale combustion applications operating on black liquor, a common side-product of the pulp industry. Here, a simplified boiler model utilizing Computational Fluid Dynamics (CFD) with Large-Eddy Simulation (LES) and Lagrangian Particle Tracking (LPT) is explored to better understand the secondary air supply system and the dispersion of sprayed droplets. The unsteady nature of the air jets and droplet dispersion in such context advocates the usage of scale-resolving simulations such as LES. In the present exploratory study, the usage of LES in recovery boiler air jet simulations is piloted for the first time. The air supply system is modeled as high-momentum-flux jets injected to a uniform cross-flow. First, the set-up was verified by performing a mesh sensitivity study. The main observed global flow features included the mixing zones, wall jet and jet impact regions, jet bending in cross-flow, and reverse flow downstream of the jets. Two different engineering relevant air supply systems, a staggered and an in-line configuration, were studied and compared in terms of mixing and droplet dispersion. The main findings of the present study are as follows. First, out of the two studied configurations, the staggered one was observed to provide a more uniform downstream temperature field. Second, the in-line configuration was noted to outperform the staggered configuration in droplet dispersion by having 22% less spray-wall impingement and 15% more droplets landing at the bottom of the furnace. Third, different modes for droplet trajectories were identified based on the global flow structures.
Reactive cooling simulation of electronic components
(2023-06-25) Zhang, Kai; Laitinen, Alpo; Shen, Yazhou; Vuorinen, Ville; Duwig, Christophe
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Low-grade heat recovery is an indispensable solution towards high energy efficiency of power electronics. The fast pace of sustainable digitalisation calls for developing alternative solutions to create a sustainable loop for decreasing the energy footprint. However, heat transfer under low-temperature differences challenges effective heat recovery processes. Therefore, in this paper, reactive fluid performance in a practical heat exchanger is investigated using high-fidelity finite rate chemistry method, which is a key step to deploy the attractive Ericsson cycle for low-temperature heat-to-electricity conversion. Under fixed thermal efficiency, it is found that replacing non-reactive fluid by N2O4 reactive fluid can immediately boost electrical efficiency of an Ericsson cycle by at least 260%. The needed primary heat exchanger component in an integrated cooling and power electronic system can be 54.8% smaller in volume whilst enabling a 26% higher thermal performance, provided that the hot source temperature is low (<403 K). For thermal processes involving high temperature hot source, substantial limitation of chemical reaction rate on the effectiveness of an Ericsson cycle is identified. Remarkably, low temperature difference is not a limitation for reactive heat transfer that continuous endo-/exothermic reaction happening throughout a heat exchanger improves Nusselt number Nu = 7.5 by a factor of ∼ 1.3 than the corresponding value (Nu = 5.9) for non-reactive fluid. Turbulence is found beneficial for reactive heat transfer, suggesting the use of corrugated-type heat exchangers for better thermal exchange rates.
Safe and sustainable design of composite smart poles for wireless technologies
(2020-10-28) Vito, Donato Di; Kanerva, Mikko; Järveläinen, Jan; Laitinen, Alpo; Pärnänen, Tuomas; Saari, Kari; Kukko, Kirsi; Hämmäinen, Heikki; Vuorinen, Ville
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The multiplicity of targets of the 5G and further future technologies, set by the modern societies and industry, lacks the establishment of design methods for the highly multidisciplinary application of wireless platforms for small cells. Constraints are set by the overall energy concept, structural safety and sustainability. Various Smart poles and Light poles exist but it is challenging to define the design drivers especially for a composite load-carrying structure. In this study, the design drivers of a composite 5G smart pole are determined and the connecting design between finite element modelling (FEM), signal penetration and computational fluid dynamics (CFD) for thermal analysis are reported as an interdisciplinary process. The results emphasize the significant effects of thermal loading on the material selection. The physical architecture, including various cutouts, is manipulated by the needs of the mmW radios, structural safety and the societal preferences of sustainable city planning, i.e., heat management and aesthetic reasons. Finally, the paint thickness and paint type must be optimized due to radome-integrated radios. In the future, sustainability regulations and realized business models will define the cost-structure and the response by customers.
Superspreading of SARS-CoV-2 at a choir rehearsal in Finland—A computational fluid dynamics view on aerosol transmission and patient interviews
(2024-09) Matvejeff, Anna Tuhkuri; Laitinen, Alpo; Korhonen, Marko; Oksanen, Lotta Maria; Geneid, Ahmed; Sanmark, Enni; Vuorinen, Ville
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Introduction COVID-19 pandemic has highlighted the role of aerosol transmission and the importance of superspreading events. We analyzed a choir rehearsal in November 2020, where all participants, except one who had recently earlier recovered from COVID-19, were infected. We explore the risk factors for severe disease in this event and model the aerosol dispersion in the rehearsal room. Materials and methods Characteristics of participants were collected by interviews and supplemented with patient records. A computational simulation of aerosol distribution in the rehearsal room and the efficacy of potential safety measures was conducted using the Large-Eddy Simulation approach. Infection risk was studied by analyzing quanta emission and exposure with the Wells-Riley equation. Results The simulation showed that airborne transmission likely explains this mass contagion event. Every singer was exposed to the virus in only 5 min from the beginning of the rehearsal, and maximum concentration levels were reached at 20 min the concentration levels started to approach a steady state after 20 min. Although concentration differences existed in the room, risk levels near (1 m) and far (5 m) from the aerosol source were similar for certain singers. Modeling indicated infection risk levels of 70–100% after one hour; the risk would have been considerably reduced by wearing high-filtration respirators. Age and pre-existing comorbidities predicted more severe disease. The high incidence of illness may be partly attributed to the relatively high median age of individuals. Additionally, those admitted to the hospital had multiple underlying health conditions that predispose them to more severe disease. Conclusions Airborne transmission and indoor space can explain this mass exposure event. High-filtration respirators could have prevented some infections. The importance of safety distances diminishes the longer the indoor event. The concept of safety distance is challenging, as our study suggests that long range airborne transmission may occur in indoor events with extended duration. We encourage informing the public, especially persons at risk, of safety measures during epidemics.
Terveydenhuollon potilasvastaanottohuoneen ilmanvaihtoratkaisut hengitystieinfektioiden torjunnassa käyttäen 3D virtaussimulointia
(2022-10-10) Luoto, Anni
Insinööritieteiden korkeakoulu | Master's thesis
Tämä työ on tehty osana E3 (Excellence in Pandemic Response and Enterprise So- lutions Co-Innovation) -hanketta Granlund Oy:n ja Business Finlandin rahoitta- mana. Käynnissä oleva pandemian aikana on arvioitu tutkimusten perus- teella Sars-CoV-2 viruksen leviävän ilmavälitteisesti hengitysaerosolien mu- kana. Pandemian ilmalevitteisyyden takia on erittäin tärkeää ymmärtää ae- rosolien liikkeet sisäilman ilmavirtojen ja ilmanvaihdoista aiheutuvien vir- tausten mukana. Työn tarkoituksena oli simuloida laskennallisella virtausmekaniikalla poistoilmaventtiilien erilaisia sijainteja potilasvastaanottohuoneessa, missä poistoilmapäätelaitteiden sijainteja vaihdeltiin. Huoneessa oli kaksi ihmistä, joista toisen ihmisen hengitystä mallinnettiin aerosolien liikkeinä huoneessa passiivisena skalaarina. Simuloinnissa mallinnettiin turbulenttista virtausta k-ω SST -turbulenssimallin avulla. Tuloilman virtauksen Reynoldsin luku oli 3333. Simulointien tuloksissa analysointiin poistoilmaventtiilin sijaintien vaih- telun vaikutus virtauskenttiin. Lopuksi simulointien passiivisen skalaarin kenttiä vertailtiin ilmanvaihtoratkaisuiden välillä. Passiivinen skalaari ku- vasti tässä aerosolien leviämistä ilmavirtojen mukana. Vertailulla saatiin va- littua toimivin ratkaisu. Toimivimmassa ratkaisussa passiivisen skalaarin pi- toisuudet olivat mahdollisimman alhaiset ihmisen hengitysalueen kohdalla. Toimivimman ratkaisun virtausrakenteista huomattiin poistoventtiilin ve- don ja ihmisen nosteen vaikutuksen yhdistyminen ilman poistumiseen huo- neesta.
A three-dimensional conjugate heat transfer model for methanol synthesis in a modular millireactor
(2022-08-31) Izbassarov, Daulet; Nyári, Judit; Laitinen, Alpo; Laari, Arto; Santasalo-Aarnio, Annukka; Vuorinen, Ville
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
In this work, a modular millireactor (MMR) is designed and modeled using the computational fluid dynamics (CFD) tool OpenFOAM. First, the method is validated against a conventional packed bed reactor (PBR) model (1D) with Aspen Plus. Next, the method is applied to study the effects of pressure (2–6 MPa) and temperature (483–533 K) on the performance of the MMR. Conjugate heat transfer (CHT) CFD results for the MMR are compared against a corresponding PBR at isothermal conditions. For the MMR, the methanol yield is shown to vary between 9–23 % within the studied parameter range. Overall, the MMR outperforms the PBR at conditions studied in this work. The maximum difference in methanol yield between MMR and the PBR is noted to be a factor of 1.71 at 533 K and 5 MPa. Such a large discrepancy advocates the usage of 3D CHT/CFD.