3,230 research outputs found
Condensation of superheated vapor of R410A and R407C inside plate heat exchangers: Experimental results and simulation procedure
No full textThis paper presents experimental results on partial condensation of 15 K superheated vapors of R407C and R410A inside two Brazed Plate Heat Exchanger (BPHE) prototypes with different geometrical characteristics, plate designs and aspect ratios. The condensation heat transfer coefficients have been measured at constant inlet saturation temperature of 41.8 °C and 36.5 °C for R407C and R410A, respectively. The refrigerant mass velocity has been varied from 15 to 40 kg m−2 s−1 whereas the outlet vapour quality between 0.01 and 0.58. The experimental results have also been compared with those previously measured by Mancin et al. (2011) during condensation of R407C and R410A inside a BPHE prototype with different plate dimensions and two refrigerant channels. Furthermore, the condensation heat transfer coefficients have been used to validate a new model recently proposed by Mancin et al. (2011), which can be used to simulate the condensation process through the BPHE
R32 Partial Condensation Inside A Brazed Plate Heat Exchanger
No full textIt is well known that when a superheated gas reaches a cold surface, the condensation starts immediately only if the wall temperature is lower than the dew temperature of the refrigerant; in this case, the heat transfer phenomenon can be also affected by the superheating temperature. This paper presents the experimental measurements of the heat transfer coefficient carried out during partial condensation of superheated R32 refrigerant in a commercial brazed plate heat exchanger prototype. The present work aims at highlighting the effects of the superheating during the partial condensation of R32 by imposing 5, 10, 15, and 25 K of vapour superheating at the inlet of the brazed plate heat exchanger prototype. The experimental measurements were carried out by varying the specific mass velocity between 15 and 40 kg m-2 s-1 and the outlet vapour quality between 0.0 and 0.65. The experimental data were collected at around 36.5 °C saturation temperature (saturation pressure of 2.27 MPa). The present data is used to validate a new step-by-step model for the calculation of the heat transfer coefficient, which accounts for the different superheating inlet temperatures
Experimental Measurements Of Air Forced Convection Through Copper Foams
No full textThis paper aims at investigating the air heat transfer and fluid flow through open-cells copper foam samples with different number of pores per unit of length (PPI) with constant porosity (ε=0.93) and foam core height of 40 mm. The experimental heat transfer coefficient and pressure drop measurements were carried out during air forced convection through electrically heated copper foams; the data points are collected in a dedicated test rig. The experimental measurements permit to understand the effects of the pore density on the heat transfer and fluid flow performance of the foams. Present data relative to copper foam samples are compared against present authors experimental measurements for 40 mm high aluminum foams at the same operative test conditions. The paper presents experimental heat transfer coefficients, pressure gradients, permeability, inertia and drag coefficients; moreover, it also reports two meaningful parameters: the normalized mean wall temperature and the pumping power per area density that permit to compare different enhanced surfaces, which can be considered suitable for electronic thermal management
"Scambio termico e fluidodinamica durante i deflussi bifase e monofase su superfici estese e in microgeometrie" "Two-phase and single-phase heat transfer and fluid flow through enhanced surfaces and in microgeometries"
No full textThe heat transfer between two fluids or between a fluid and a surface, during both single-phase and two-phase flow, is intimately linked to the geometrical characteristic of the heat transfer surface involved. Generally speaking, if the heat transfer area increases, the global heat transfer also increases, but if the surface is developed to improve heat transfer, the overall performance can be higher.
Compact and efficient heat exchangers and heat sinks are more and more demanded for electronics cooling applications, refrigeration and air conditioning systems. For this reason, the development of new enhanced surfaces is a critical issue of thermal research.
In this Ph. D. thesis, two arguments connected with enhanced heat transfer surfaces have been experimentally and analytically studied. In particular, the condensation of R410A inside a microfin tube and air forced convection through different enhanced surfaces have been analysed.
With respect to two-phase flow, the heat transfer coefficients and the pressure drops during the condensation of R410A at 40 °C of saturation temperature inside a microfin tube have been measured. The experimental analysis have been carried out at the Dipartimento di Fisica Tecnica of the University of Padova.
Using a database of 4000 experimental data points, a new correlation for the computation of the heat transfer coefficient during the condensation of several natural and synthetic refrigerants inside different enhanced microfinned tubes has been developed.
With respect to single-phase flow, a new experimental test rig has been developed, designed and built at the laboratory of the Dipartimento di Fisica Tecnica of the University of Padova. This test rig allows to carry out accurate heat transfer and pressure drop measurements during air forced convection through different enhanced surfaces, such as traditional finned surfaces, metal foams and microgeometries, etc.
The apparatus has been calibrated by testing a traditional finned surface for electronics cooling applications. Then, the heat transfer coefficients and the pressure drops during air forced convection through five different aluminum foams have been measured. All tested samples have been heated by imposing different heat fluxes, which have been varied between 150 W and 400 W. The experimental results have been compared with those obtained for the traditional finned surface.Lo scambio termico tra due fluidi o tra un fluido e una superficie, sia esso monofase o bifase, è profondamente legato alle caratteristiche geometriche delle superfici di scambio con le quali i fluidi stessi sono in contatto. Aumentare la superficie di scambio termico significa, in generale, incrementare lo scambio termico globale. Se poi questa superficie viene sviluppata per migliorare l’efficienza dello scambio termico, le prestazioni possono crescere ulteriormente. Il raffreddamento di componenti elettronici, la refrigerazione e il condizionamento dell’aria richiedono scambiatori di calore sempre più efficienti e compatti, pertanto lo sviluppo di nuove superfici di scambio termico risulta essere un obiettivo fondamentale della ricerca.
In questo lavoro di tesi sono stati affrontati, sia sperimentalmente che analiticamente, due argomenti connessi allo scambio termico su superfici estese. In particolare, si sono studiati la condensazione all’interno di tubi micro-alettati e la convezione forzata di aria su schiume metalliche e su superfici alettate tradizionali. Per quanto riguarda lo scambio termico bifase, sono stati misurati i coefficienti di scambio termico e le perdite di carico durante la condensazione di R410A all’interno di un tubo micro-alettato, alla temperatura di saturazione di 40 °C. Le prove sono state realizzate utilizzando l’impianto presente presso il Dipartimento di Fisica Tecnica dell’Università di Padova.
Successivamente, utilizzando un database contenente più di 4000 dati sperimentali, è stato sviluppato un nuovo modello per la stima del coefficiente di scambio termico durante la condensazione di refrigeranti naturali e sintetici all’interno di tubi micro-alettati.
Con riferimento allo scambio termico monofase, un nuovo impianto sperimentale è stato sviluppato, progettato e costruito presso il Dipartimento di Fisica Tecnica dell’Università di Padova. Questo apparato permette di eseguire accurate misure di scambio termico e perdite di carico durante la convezione forzata di aria attraverso superfici estese, quali tradizionali superfici alettate, schiume metalliche, microgeometrie, ecc. L’impianto sperimentale è stato calibrato testando una superficie alettata tradizionale per il raffreddamento di componenti elettronici. Successivamente, sono stati misurati i coefficienti di scambio e le perdite di carico di cinque schiume di alluminio durante il deflusso di aria. Tutti i provini sono stati riscaldati elettricamente imponendo differenti flussi temici tra 150 W e 400 W. I risultati sperimentali sono stati confrontati con quelli ottenuti per la superficie alettata tradizionale
Nano-PCMs for enhanced energy storage and passive cooling applications
No full textIt is well known that the heat transfer associated with a phase change process is much higher than sensible enthalpy change even in forced convection. In particular, the vaporization process has been widely studied because it exploits the highest heat transfer coefficient; this heat transfer mechanism is used in both passive (i.e. heat pipes) and active (i.e. refrigerating machines) cooling devices. However, the solid liquid phase change process is another interesting possibility to reject even high heat loads, especially when they are intermittent. The term Phase Change Materials (PCMs) commonly refers to those materials, which use the solid-liquid phase change process to adsorb and then release heat loads (Mancin et al., 2015). The present work aims at investigating the feasibility of a new challenging use of Aluminum Oxide (Al2O3) and Carbon Black (CB) nanoparticles to enhance the thermal properties: thermal conductivity, specific heat, and latent heat of pure paraffin waxes to obtain a new class of PCMs, the so-called nano-PCMs. The nano-PCMs were obtained by seeding 1 wt% of nanoparticles in paraffin waxes with melting temperatures of 20 C and 25 C. The thermophysical properties were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCM. These new nano-PCMs can represent a feasible and interesting way to mitigate or eliminate the intrinsic limitations in the use of paraffin waxes as PCMs for both energy storage and passive cooling applications
Reversible Heat Pump Coupled with Ground Ice Storage for Annual Air Conditioning: An Energy Analysis
No full textDuring annual operation, a heat pump produces both heating and cooling effects, so it would be of great advantage to store one of the two to be then used when it is necessary. To do this, a seasonal energy storage is necessary. This paper presents results relative to the use of a ground ice thermal energy storage (I-TES) integrated with a reversible heat pump for annual air conditioning. The energy analysis is based on heating and cooling loads for a residential building located in Milan. In particular, the focus is on the most important parameters affecting the performance of both the whole system and the Ice Tank, which is the position and the thickness of the insulation layers and the shape of the ice tank. A biannual simulation of the system allows for a full description of the ice tank behavior during the charging and discharging processes. The main objective of the study is to suggest a first tentative procedure to design the I-TES integrated system with the best energy performance
An assesment on forced convection in metal foams
No full textMetal foams are a class of cellular structured materials with open cells randomly
oriented and mostly homogeneous in size and shape. In the last decade, several authors have
discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for
air conditioning, refrigeration, and electronic cooling applications. This paper reports an
assessment on the forced convection through metal foams presenting experimental and
analytical results carried out during air heat transfer through twelve aluminum foam samples
and nine copper foam samples. The metal foam samples present different numbers of pores per
linear inch (PPI), which vary between 5 and 40 with a porosity ranging between 0.896-0.956;
samples of different heights have been studied. From the experimental measurements two
correlations for the heat transfer coefficient and pressure drop calculations have been
developed. These models can be successfully used to optimize different foam heat exchangers
for any given application
Shell and tube carbon dioxide gas coolers - Experimental results and modelling
No full textThis paper experimentally compares the heat transfer performance of four different shell and tube gas coolers implemented in a 5 kW, R744 water/water heat pump controlled by a back pressure valve as expansion device. The tubes bundle consists of 10 tubes in a 30° arrangement for all the gas coolers with different tube geometries: smooth, corrugated, internally grooved, and corrugated and internally grooved, respectively. The results were carried out at fixed gas cooler inlet water temperature of around 25 °C and by imposing two inlet gas cooling pressures: 8 MPa and 10 MPa, and by varying the water flow rate from 340 to 786 l h-1. A step-by-step procedure for the simulation of the heat transfer cooling process of the carbon dioxide in shell and tubes gas coolers is proposed and validated, allowing for a direct comparison of the heat transfer performance of the shell and tube gas coolers
Effects of lubricant on carbon dioxide heat transfer in transcritical refrigerating cycles
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