1,720,961 research outputs found
La generazione del freddo mediante l'uso di onde acustiche: storia e sviluppi
No full textIn questa rassegna viene descritto lo sviluppo dei dispositivi termoacustici, che possono produrre il trasferimento di calore da una sorgente termodinamica a un’altra, a partire dai primi esperimenti di Rayleigh, che ha correttamente fornitola spiegazione della trasformazione inversa del calore in energia delle onde elastiche, fino agli attuali apparati esistenti dalle prestazioni rilevanti. C’è stata e continua a esserci una straordinaria quantità di progetti e di realizzazioni, relativamente sia ai motori termodinamici che ai frigoriferi acustici, al fine di sfruttare i vantaggi e la sostenibilità di questi tipi di prodotti: assenza di parti meccaniche in movimento e utilizzo di gas non inquinanti. La linea di sviluppo seguita in questa relazione è finalizzata alla spiegazione dei processi fisici fondamentali che sono alla base dei dispositivi sperimentali prodotti nel settore della refrigerazione. Vengono inizialmente descritti i tubi di Soundhauss e di Rijke, in cui il calore prodotto da una fiamma può generare un suono intenso, la cui interpretazione è legata alla relazione di fase tra il ciclo dinamico di una particella di gas in un campo acustico e il trasferimento di calore a una sorgente esterna. Viene poi dettagliatamente illustrato il motore Stirling, punto focale dei successivi sviluppi, dove un rigeneratore, che è da considerarsi l’elemento fondamentale per il processo di produzione del freddo, viene introdotto tra le sorgenti termiche. Un miglioramento naturale del dispositivo basato sul ciclo Stirling è il refrigeratore pulse tube, in cui il pistone d’espansione viene adeguatamente sostituito da un tubo cilindrico non in movimento. Viene successivamente fornita una presentazione dei due tipi effettivamente esistenti di frigoriferi termoacustici, che si basano o su un'onda acustica progressiva o su una stazionaria. Infine, sono descritti gli sviluppi dei dispositivi di più recente progettazione, le cui prestazioni vengono dovutamente riportate, fino al sorprendente risultato del raggiungimento di 1.2K nel campo della criogenia.
The development of thermoacoustic devices that may produce the transfer of heat from one thermodynamic source to another one is described in this review from the very first experiments produced by Rayleigh, who did really explained the reverse transformation of heat into elastic wave energy, to the actual highly performing apparatuses. An extraordinary quantity of projects and realizations have been and are actually being performed relative to both thermodynamic engines and acoustic refrigerators, in order to exploit the advantages and sustainability of these kinds of products: no mechanical moving parts and use of not polluting gases. A line of development is followed in this report, aimed at the explanation of the fundamental physical processes underlying the experimental devices produced in the refrigeration field. Soundhauss and Rijke tubes are described at the beginning, where heat produced by a flame may generate an intense sound, whose interpretation is based on the phase relation of the dynamical cycle of a gas particle in an acoustic field and its heat exchange with an external source. The Stirling engine is then described in detail, as the focal point of successive developments, where a regenerator - which is to be considered as the basic elements for the process of cold production - is introduced between the thermal sources. A natural improvement of the Stirling device is the pulse tube refrigerator, where the piston which causes the expansion of the gas is properly substituted by a not moving tube element. Presentation is successively given of the two actually existing types of thermoacoustic refrigerators, which rely either on a traveling acoustic wave or on a stationary one. Finally, the developments of the most recently designed devices are described, and the performances duly reported, up to the astonishing result of the attainment of 1.2K in the field of cryogenics.The development of thermoacoustic devices that may produce the transfer of heat from one thermodynamic source to another one is described in this review from the very first experiments produced by Rayleigh, who did really explained the reverse transformation of heat into elastic wave energy, to the actual highly performing apparatuses. An extraordinary quantity of projects and realizations have been and are actually being performed relative to both thermodynamic engines and acoustic refrigerators, in order to exploit the advantages and sustainability of these kinds of products: no mechanical moving parts and use of not polluting gases. A line of development is followed in this report, aimed at the explanation of the fundamental physical processes underlying the experimental devices produced in the refrigeration field. Soundhauss and Rijke tubes are described at the beginning, where heat produced by a flame may generate an intense sound, whose interpretation is based on the phase relation of the dynamical cycle of a gas particle in an acoustic field and its heat exchange with an external source. The Stirling engine is then described in detail, as the focal point of successive developments, where a regenerator - which is to be considered as the basic elements for the process of cold production - is introduced between the thermal sources. A natural improvement of the Stirling device is the pulse tube refrigerator, where the piston which causes the expansion of the gas is properly substituted by a not moving tube element. Presentation is successively given of the two actually existing types of thermoacoustic refrigerators, which rely either on a traveling acoustic wave or on a stationary one. Finally, the developments of the most recently designed devices are described, and the performances duly reported, up to the astonishing result of the attainment of 1.2K in the field of cryogenics
Aerodynamic noise within hyperbaric chambers: assessment and possible mitigation
No full textThe use of hyperbaric chambers prevailed in medical therapy for vascular diseases and for outbreaks of embolism in emergency situations. This implies the achievement of environmental pressure conditions from two to three times higher than the atmospheric one, in such a short time to require delivery of the air flow that leads to very high intensity noise. The attenuation of this noise is usually performed through the installation of silencers at the air outlets. In this work a series of noise measurements is presented, carried out inside the chamber, limited to the periods it is available to maintenance activities, in various conditions of the opening of the air inlet valve, with and without silencers mounted on the air outlet nozzles and with or without different layers of floor covering. In order to characterize the attenuation capacity of the silencers and to verify that the air flow is the most significant source of noise, a series of noise measurements is also presented, carried out in the open field while emptying a pressurized tank, in various conditions of opening of the exhaust valve, for different types of silencers placed at the outlet. The analysis of the frequency spectra allows to evaluate the behaviour of these devices, at the light of theoretical predictions concerning the aerodynamic noise. The results suggest that the optimization of some of the flow parameters and the operating conditions of the chamber can contribute significantly to the reduction of noise, with a consequent net improvement of such device, in particular under operating conditions of emergency
Analisi sperimentale di silenziatori commerciali per la riduzione del rumore aerodinamico all’interno di camere iperbariche
No full textL’uso delle camere iperbariche, invalso in terapie medicali per patologie vascolari e in situazioni di emergenza per insorgenze di embolia, prevede il raggiungimento di una condizione di pressione ambientale da due a tre volte superiore a quella atmosferica, in tempi così ridotti da richiedere un’erogazione del flusso d’aria tale da ingenerare rumore di elevata intensità. L’attenuazione di tale rumore è in genere affidata all’installazione di silenziatori in corrispondenza delle bocche di uscita dell’aria. In questo lavoro viene presentata una serie di misure di rumore effettuate in campo aperto durante lo svuotamento di un serbatoio pressurizzato, in varie condizioni di apertura della valvola di scarico, per differenti tipologie di silenziatori posti sulla bocca di uscita. L’analisi degli spettri in frequenza ottenuti permette una preliminare valutazione del comportamento di tali dispositivi, alla luce delle previsioni teoriche relative al rumore aerodinamico. I risultati preliminari ottenuti inducono oggi a ritenere che l’ottimizzazione di alcuni dei parametri di flusso e delle condizioni di funzionamento della camera possa contribuire sensibilmente alla riduzione del rumore e a un conseguente netto miglioramento di tale dispositivo, in particolare in condizioni operative di emergenza. La scelta dei dispositivi di attenuazione dovrà essere attentamente valutata in relazione agli stringenti vincoli di sicurezza vigenti.The use of hyperbaric chambers prevailed in medical therapy for vascular diseases and for outbreaks of embolism in emergency situations. This implies the achievement of environmental pressure conditions from two to three times higher than the atmospheric one, in such a short time to require delivery of the air flow that leads to very high intensity noise. The attenuation of this noise is usually performed through the installation of silencers at the air outlets. In this work a series of noise measurements is presented, carried out inside the chamber, limited to the periods it is available to maintenance activities, in various conditions of the opening of the air inlet valve, with and without silencers mounted on the air outlet nozzles and with or without different layers of floor covering. In order to characterize the attenuation capacity of the silencers and to verify that the air flow is the most significant source of noise, a series of noise measurements is also presented, carried out in the open field while emptying a pressurized tank, in various conditions of opening of the exhaust valve, for different types of silencers placed at the outlet. The analysis of the frequency spectra allows to evaluate the behaviour of these devices, at the light of theoretical predictions concerning the aerodynamic noise. The results suggest that the optimization of some of the flow parameters and the operating conditions of the chamber can contribute significantly to the reduction of noise, with a consequent net improvement of such device, in particular under operating conditions of emergency
Cap. 9 Termoacustica
No full textIn questo capitolo, dopo una prima sezione dedicata a evidenziare i principali eventi storici di interesse per l’evoluzione della termoacustica, sono illustrati i dispositivi a onda stazionaria eccitata all’interno di tubi risonanti e, successivamente, quelli a onda progressiva. Alla descrizione di questi ultimi è premessa una sezione relativamente estesa sul refrigeratore di Stirling, inteso come dispositivo di riferimento per la comprensione della fenomenologia di base, nel quale la dinamica del gas refrigerante è attuata dal moto di un pistone piuttosto che indotta direttamente dalla propagazione dell’onda elastica. Segue una sezione di sintesi sulle possibili applicazioni dei dispositivi e sulle realizzazioni ottenute, con un interesse particolare alle potenzialità più specifiche dei refrigeratori acustici rispetto agli altri tipi disponibili.In this chapter, the first section concerns the main historical events of interest for the development of the thermoacoustics. Then, standing wave devices are described, within the resonant tubes. After a section on the Stirling chiller, useful to understand the basic phenomenology, progressive wave devices are described. The last section summarizes possible applications of the devices and the achievements obtained
Temperature di raffreddamento di un frigorifero acustico in funzione della potenza di eccitazione
No full textUn piccolo frigorifero acustico basato su un risuonatore a quarto d’onda, costituito da un tubo a sezione cilindrica chiuso a un’estremità, aperto all’altra e riempito d’aria alla pressione
atmosferica, eccitato da un altoparlante posto in prossimità dell’estremità aperta, è stato usato per studiare la variazione delle temperature di raffreddamento in funzione della frequenza
dell’onda acustica e del tempo di applicazione di quest’ultima. Le misure sono state effettuate per frequenze variabili nell’intorno della frequenza nominale di risonanza del tubo pari a
385 Hz. Sono stati misurati, a parità di potenza acustica, i valori massimi delle temperature di raffreddamento e il tempo necessario per il loro raggiungimentoA small acoustic refrigerator based on a quarter-wave resonator, composed of a tube of cylindrical section closed at one end and open at the other one, filled with air at atmospheric pressure, excited from a speaker placed in proximity to the open end, has been used to study the variation of the cooling temperatures as a function of the frequency of the acoustic wave and of the time of application of the latter. The measurements were carried out for frequencies varying around the nominal resonant frequency of the tube equal to 385 Hz. The maximum values of cooling temperatures and the time required to achieve them were measured with equal acoustic power
Rumore aerodinamico all'interno di camere iperbariche
No full textL’uso delle camere iperbariche, invalso in terapie medicali per patologie vascolari e in situazioni di emergenza per insorgenze di embolia, prevede il raggiungimento di una condizione di pressione ambientale da due a tre volte superiore a quella atmosferica, in tempi così ridotti da richiedere un’erogazione del flusso d’aria tale da ingenerare rumore di elevata intensità. L’analisi teorica di un flusso omogeneo in uscita su un ambiente stazionario prevede una dipendenza dell’immissione del suono dall’ottava potenza della velocità del flusso e uno spostamento corrispondente dell’intensità massima verso le più alte frequenze. In questo lavoro viene presentata una serie di misure di rumore effettuate all’interno di una camera, in varie condizioni di apertura della valvola di immissione dell’aria e per differenti posizioni dei rilevatori, in presenza o meno dei silenziatori sulla bocca di uscita e delle strutture di pavimentazione. Si descrive inoltre un modello di flusso realizzato per interpretare i risultati sperimentali alla luce delle previsioni teoriche del rumore aerodinamico. I risultati preliminari ottenuti inducono oggi a ritenere che l’ottimizzazione di alcuni dei parametri di flusso e delle condizioni di funzionamento della camera possa contribuire sensibilmente alla riduzione del rumore e a un conseguente netto miglioramento di tale dispositivo, in particolare in condizioni operative di emergenza.The use of hyperbaric chambers prevailed in medical therapy for vascular diseases and for outbreaks of embolism in emergency situations. This implies the achievement of environmental pressure conditions from two to three times higher than the atmospheric one, in such a short time to require delivery of the air flow that leads to very high intensity noise.. In this work a series of noise measurements is presented, carried out inside the chamber, limited to the periods it is available to maintenance activities, in various conditions of the opening of the air inlet valve, with and without silencers mounted on the air outlet nozzles and with or without different layers of floor covering.. The analysis of the frequency spectra allows to evaluate the behaviour of these devices, at the light of theoretical predictions concerning the aerodynamic noise. The results suggest that the optimization of some of the flow parameters and the operating conditions of the chamber can contribute significantly to the reduction of noise, with a consequent net improvement of such device, in particular under operating conditions of emergency
Resonance conditions of the air column in a thermoacoustic refrigerator
No full textAmong the refrigerating fluids compatible with the environment and which do not present problems of safety appears the air, ideal as regards the safety of both as regards the availability and cost. The air is used as a refrigerant since the beginning of refrigeration applications. It presents the ODP and GWP indices equal to zero, but its use has always been penalized by the low energy performance of the Brayton-Joule inverse cycle, compared to vapor compression cycles so as to exclude its use in air conditioning systems of buildings. A cooling system with acoustic resonator uses the direct conversion of mechanical work, associated with the wave propagation, in a transfer of heat from a heat source at low temperature to a source at a higher temperature. One problem in the use of an acoustic resonator is the realization and the position of the heat exchanger within the resonant volume of air. With regard to the position in a thermoacoustic tube resonator, it is mandatory to preliminarily know what the conditions of the resonant air column are, since it gets excited by an outside power source, whose form and position determine the resonance conditions. In the present paper, a simple model is presented and the definition of an effective length of the resonating tube of a thermoacoustic refrigerator is properly given. Few data are also given relative to the temperature difference obtained in a specific experimental case
A first approach to natural thermoventilation of residential buildings through solar chimneys supplied by solar ponds
No full textThe exploitation of natural ventilation is a good solution to improve buildings from an energetic point of view and to fulfill the requirements demanded by the thermoigrometric comfort and the air quality in enclosed spaces. Some past researches demonstrated how some devices, useful to this purpose, follow the principles of solar chimneys and are able to move air masses while exploiting the Archimedes thrust. The natural ventilation must be supplied by a flow moving upward generated by a heat source performing at temperatures slightly higher than the one present in the environment. To have a minimum energetic effect, the heat can be extracted from solar ponds; solar ponds are able to collect and store solar energy in the geographical regions characterized by sufficient values of solar radiation. Thus it is possible, in summer, to provoke a nocturnal natural ventilation useful for the air change in indoor spaces (in those climatic areas where, during the night, there is a temperature gradient)
A first approach to natural thermoventilation of residential buildings through ventilation chimneys supplied by solar ponds
Full text linkThe exploitation of natural ventilation is a good solution to improve buildings from an energetic point of view and to fulfill the requirements demanded by the thermohygrometric comfort and the air quality in enclosed spaces. Some past researches demonstrated how some devices, useful to this purpose, follow the principles of solar chimneys and are able to move air masses while exploiting the Archimedes thrust. The natural ventilation must be supplied by a flow moving upward, generated by a heat source performing at temperatures slightly higher than the one present in the environment. To have a minimum energetic effect, the heat can be extracted from solar ponds; solar ponds are able to collect and store solar energy in the geographical regions characterized by sufficient values of solar radiation. Thus it is possible, in summer, to provoke a nocturnal natural ventilation useful for the air change in indoor spaces (in those climatic areas where, during the night, there is a temperature gradient)
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