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Flow pattern during condensation of three refrigerants: microfin vs. smooth tube
No full textThere is agreement in the open literature that the mechanisms of heat transfer and pressure drop are intimately
linked with the prevailing two-phase flow regime.
During condensation inside horizontal tubes, the two-phase flow may be dominated by vapour shear or gravity
forces. While annular flow pattern is associated with high vapour shear, stratified, wavy and slug flows appear
when gravity is the controlling force.
Very poor evidence about the effect of microfins on the flow patterns during condensation is given in the open
literature. Thus, to investigate the two phase flow pattern during condensation a specific test section was built
up; the fluid outlet pattern can be analysed and recorded.
The heat transfer and pressure drop performances of microfin tubes during condensation of refrigerants have
been studied experimentally and theoretically.
In previous papers the present authors reported their own data condensing R134a, R410A and R236ea inside two
different tubes: a 9.50 mm outer diameter microfin tube (7.69 mm inside diameter at the fin tip and 60 fins with
0.23 mm fin height and 13° helix angle) and a plain 8.00 mm inner diameter tube.
The same operative conditions (saturation temperature, vapour quality and mass flux) used for heat transfer
measurements are reproduced in the visualisation section in order to investigate the specific flow pattern for
three different refrigerants with high (R410A), intermediate (R134a) and low (R236ea) saturation pressure.
For the study of the main flow patterns, in particular focusing on the stratified/annular mode transition, the
visualisation experimental data are analysed with reference to parameters like dimensionless vapour velocity,
Martinelli parameter and void fraction, that are adopted in the main available flow pattern maps.
Since the available maps are designed only for smooth tubes, the results of this wide experimental work can be
used in order to define a new flow pattern map specifically dedicated to microfin and enhanced tubes
Experimental investigation into two-phase flow patterns inside a herringbone microfin tube
No full textDuring in-tube microfinned enhanced tubes show a heat transfer enhancement, as compared to
equivalent smooth tubes under the same operating conditions, that is partly due to the mere increase
in the effective exchange area and additionally to the turbulence induced in the liquid film by the
micro fins and to the surface tension effect on the liquid drainage.
Furthermore there is agreement in the literature that the mechanisms of heat transfer and pressure
drop are intimately linked with the prevailing two-phase flow regime.
In the recent open literature evidence is given to the effect of fins orientation on flow patterns in
herringbone tubes (Miyara et al., 2003). In particular, at the same operating conditions, it was
pointed out that when the fins convergences are positioned at the top and bottom of the tube
(dubbed here as Position-I), the occurring flow pattern can be completely different from the tube
arrangement with the fins convergences at both sides (Position-II). In a previous paper by the
present authors, the “Position-II” arrangement was investigated with three different refrigerants for
a saturation temperature of 40°C and mass velocities 100÷800 kg m-2s-1. In this paper the “Position-
I” arrangement is now investigated for the fluid R134a and a comparison with the available
visualization data for “Position-II” is presented.
In order to investigate the two phase flow pattern during condensation a specific test section was
built up. For the study of the main flow patterns, in particular focusing on the stratified/annular
mode transition, the visualisation experimental data are analysed with reference to the
dimensionless vapour velocity and the Martinelli parameter
Scambio termico e gradiente di pressione nella condensazione entro microcanali
No full textL’utilizzo di microcanali nei condensatori e negli evaporatori di impianti di condizionamento dell’aria, specialmente
nell’industria automobilistica, è ormai piuttosto diffuso, dati i vantaggi dal punto di vista sia energetico che ambientale. Infatti,
data la loro forma e le ridotte dimensioni esterne, questi condotti oppongono una minore resistenza all’aria, nonché realizzano
migliori prestazioni di scambio termico lato interno, se paragonati ai tubi tradizionali di dimensioni maggiori; inoltre,
permettono di contenere la carica di refrigerante, limitando così le emissioni nell’ambiente. Tuttavia, dal punto di vista termico
l’utilizzo di questi canali richiede uno studio specifico poichè molti fenomeni, che sappiamo essere presenti durante la
condensazione in tubi di dimensioni tradizionali (diametro interno 7-10 mm), vengono soppressi o ridotti, mentre tendono a
diventare importanti nuovi fattori, come ad esempio la tensione superficiale, che influenza in maniera rilevante il regime di
deflusso.
Gli studi presenti in letteratura sulla condensazione entro microcanali sono ancora pochi e incompleti, e nuove analisi e
ricerche sono necessarie per comprendere e descrivere appieno il fenomeno. In questo lavoro gli autori presentano in maniera
critica alcuni tra i più recenti studi sulla condensazione di fluidi frigorigeni entro microcanali, confrontando i risultati
sperimentali con modelli di previsione dei coefficienti di scambio termico e dei gradienti di pressione
Studio sperimentale sul deflusso di una soluzione LiBr-H2O irrorata su una colonna di tubi
No full textExperimental heat transfer coefficients during condensation of pure refrigerants on a commercial enhanced tube
No full textThis work experimentally investigates the effect of vapour shear on heat transfer during condensation of pure refrigerants on a commercial enhanced tube Hitachi Thermoexcel. Two different series of tests are taken: the first woth refrigerant 11, the second with refrigerants 113, with vapour pressure ranging from 100 to 190 kPa, average condensation temperature difference varying from 4 to 16°C an maximum vapour velocity from 1 to 38 m/s. The present experimental results are compared with analogous data prevously obtained by the Authors with integral-fin tubes and with the smooth tube trend
I fluidi frigorigeni. Processi di sostituzione e nuove frontiere tecnologiche
No full textQuesto studio, promosso dal Servizio Trasferimento Tecnologico di
AREA Science Park, è stato realizzato dal Dipartimento di Fisica
Tecnica dell’Università di Padova. Il tema della ricerca consiste nel
fornire criticamente lo stato attuale dell’arte nel processo di sostituzione
dei fluidi frigorigeni nelle apparecchiature di produzione del
freddo delle macchine a compressione meccanica di vapori, con riferimento
a tutte le applicazioni tecnologiche ove tali macchine sono
impiegate
Condensation heat transfer coefficients on enhanced tubes: experimental and theoretical analysis on vapour shear effects
No full textLow-GWP refrigerants flow boiling heat transfer in a 5 PPI copper foam
No full textThis paper reports an experimental investigation of the heat transfer performance of the new low-GWP refrigerants, R1234yf and R1234ze(E), during flow boiling heat transfer inside a horizontal high porosity copper foam with 5 Pores Per Inch (PPI). Metal foams are a class of cellular structured materials consisting of a stochastic distribution of interconnected pores; these materials have been proposed as effective solutions for heat transfer enhancement during both single and two-phase heat transfer. R1234yf and R1234ze(E) refrigerants are appealing alternatives of the more traditional R134a by virtue of their negligible values of GWP and normal boiling temperatures close to that of R134a, which make them suitable solution in several different applications, such as: refrigeration and air conditioning and electronic thermal management. This work compares the two-phase heat transfer behaviour of these new HFO refrigerants, studying the boiling process inside a porous medium and permitting to understand their effective heat transfer capabilities. The experimental measurements were carried out by imposing three different heat fluxes: 50, 75, and 100 kW m(-2), at a constant saturation temperature of 30 degrees C; the refrigerant mass velocity was varied between 50 and 200 kg m(-2) s(-1), whilst the mean vapour quality varied from 0.2 to 0.95. The two-phase heat transfer and pressure drop performance of the two new HFO refrigerants is compared against that of the more traditional R134a. (C) 2015 Elsevier Ltd. All rights reserved
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