This thesis investigates, through computational fluid dynamics simulations carried out with the open-source code OpenFOAM, the cooling channel of an additively manufacturable heat exchanger for cryogenic applications, in which the refrigerant R452A flows through a helical coil applied to the wall of a cylinder. Compared with the reference component, whose channel has a circular cross-section, the analysis considers a semicircular cross-section of the same diameter, 8 mm, in which the flat wall is the useful heat-transfer surface and the walls are held at three distinct temperatures. The analysis compares four geometric configurations, built by introducing one element at a time: a straight section, a planar 180° bend, a half-turn helix with an 11 mm pitch, and a full-turn helix. For each configuration, four values of equivalent wall roughness were simulated, from a smooth wall up to 800 µm, so as to represent the roughness typical of channels produced by additive manufacturing. Turbulence is described using the k-ω model coupled with wall functions for rough surfaces. The reliability of the solutions was verified through convergence criteria, mass conservation, closure of the energy balance and, for the smooth-wall straight case, comparison with classical correlations from the literature. Curvature generates two Dean vortices that shift the cold core of the fluid toward the outer wall. Compared with the straight channel, at equal roughness, the overall thermal power increases by 23% and the pressure drop by 33%, but the heat transfer on the useful surface decreases by 46%. On the useful wall the boundary layer widens to more than half of the cross-section, while on the outer walls it thins. The torsion of the helix makes the vortices asymmetric without appreciably changing the integral quantities. Along a full turn the refrigerant heats up and the heat transfer on the useful wall decreases rapidly. Roughness increases the pressure drop much more than the heat transfer and, in curved geometries, further reduces the heat exchanged by the useful surface. The results indicate that, for this component, the channel configuration should be chosen by taking into account the direction of the secondary flow relative to the useful wall.
La tesi studia, mediante simulazioni di fluidodinamica computazionale svolte con il codice di calcolo open source OpenFOAM, il canale di raffreddamento di uno scambiatore di calore per applicazioni criogeniche producibile mediante fabbricazione additiva, nel quale il refrigerante R452A percorre una serpentina elicoidale applicata alla parete di un cilindro. Rispetto al componente di riferimento, nel quale il canale ha sezione circolare, l’analisi considera una sezione semicircolare dello stesso diametro, 8 mm, nella quale la parete piatta costituisce la superficie utile allo scambio e le pareti sono mantenute a tre temperature distinte. L’analisi confronta quattro configurazioni geometriche, costruite introducendo un elemento alla volta: un tratto rettilineo, una curva piana di 180°, un’elica a mezzo giro con passo di 11 mm e un’elica a giro completo. Per ciascuna configurazione sono stati simulati quattro valori della rugosità equivalente di parete, dalla parete liscia a 800 µm, in modo da rappresentare la rugosità caratteristica dei canali ottenuti per fabbricazione additiva. La turbolenza è descritta con il modello k-! accoppiato a funzioni di parete per superfici rugose; l’affidabilità delle soluzioni è stata verificata attraverso i criteri di convergenza, la conservazione della massa, la chiusura del bilancio energetico e, per il caso rettilineo a parete liscia, il confronto con le correlazioni classiche della letteratura. La curvatura genera due vortici di Dean che spostano il nucleo freddo del fluido verso la parete esterna: rispetto al canale rettilineo, a parità di rugosità, la potenza complessiva aumenta del 23% e la perdita di carico del 33%, ma lo scambio sulla superficie utile cala del 46%. Sulla parete utile lo strato limite si allarga fino a più di metà sezione, sulle pareti esterne si assottiglia. La torsione dell’elica rende asimmetrici i vortici senza modificare in modo apprezzabile le grandezze integrali. Lungo un giro completo il refrigerante si scalda e lo scambio sulla parete utile diminuisce rapidamente. La rugosità aumenta la perdita di carico molto più dello scambio termico e, nelle geometrie curve, riduce ulteriormente il calore scambiato dalla superficie utile. I risultati indicano che, per questo componente, la configurazione del canale va scelta tenendo conto della direzione del flusso secondario rispetto alla parete utile.
STUDIO DELLA CONFIGURAZIONE GEOMETRICA DI UNO SCAMBIATORE DI CALORE PER APPLICAZIONI CRIOGENICHE PRODUCIBILE MEDIANTE ADDITIVE MANUFACTURING
FRERES, FRANCESCO
2025/2026
Abstract
This thesis investigates, through computational fluid dynamics simulations carried out with the open-source code OpenFOAM, the cooling channel of an additively manufacturable heat exchanger for cryogenic applications, in which the refrigerant R452A flows through a helical coil applied to the wall of a cylinder. Compared with the reference component, whose channel has a circular cross-section, the analysis considers a semicircular cross-section of the same diameter, 8 mm, in which the flat wall is the useful heat-transfer surface and the walls are held at three distinct temperatures. The analysis compares four geometric configurations, built by introducing one element at a time: a straight section, a planar 180° bend, a half-turn helix with an 11 mm pitch, and a full-turn helix. For each configuration, four values of equivalent wall roughness were simulated, from a smooth wall up to 800 µm, so as to represent the roughness typical of channels produced by additive manufacturing. Turbulence is described using the k-ω model coupled with wall functions for rough surfaces. The reliability of the solutions was verified through convergence criteria, mass conservation, closure of the energy balance and, for the smooth-wall straight case, comparison with classical correlations from the literature. Curvature generates two Dean vortices that shift the cold core of the fluid toward the outer wall. Compared with the straight channel, at equal roughness, the overall thermal power increases by 23% and the pressure drop by 33%, but the heat transfer on the useful surface decreases by 46%. On the useful wall the boundary layer widens to more than half of the cross-section, while on the outer walls it thins. The torsion of the helix makes the vortices asymmetric without appreciably changing the integral quantities. Along a full turn the refrigerant heats up and the heat transfer on the useful wall decreases rapidly. Roughness increases the pressure drop much more than the heat transfer and, in curved geometries, further reduces the heat exchanged by the useful surface. The results indicate that, for this component, the channel configuration should be chosen by taking into account the direction of the secondary flow relative to the useful wall.| File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.14253/110678