147 research outputs found
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
Flow patterns during condensation of refrigerants inside enhanced tubes
No full textEnhanced tubes have already been widely used for air-conditioning and refrigeration applications as
they ensure a large heat transfer enhancement with a relatively low pressure drop increase. During
condensation enhanced tubes show a heat transfer enhancement, 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 enhanced surface
(fins) and to the surface tension effect on the liquid drainage.
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.
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. In a fully developed annular flow
pattern, there is a thin condensate film on the entire tube wall, while the gas phase flows in the
central core, and heat transfer is governed by vapour shear and turbulence. Very poor evidence
about the effect of microfins both in helical and “herringbone” shapes on flow patterns during
condensation is given in the open literature. Thus, to investigate the two phase flow pattern during
condensation, a special test section was built. Experimental observations for herringbone tube with
three fluids (R236ea, R134a, R410A) in a wide range of operative conditions (mass flux and vapour
quality) are reported in this paper.
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 and Martinelli parameter, that are commonly used in most available
flow pattern maps
Experimental Heat Transfer Coefficients during External Condensation of Halogenated Refrigerants on Enhanced Tubes
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
Update on condensation heat transfer and pressure drop inside minichannels
No full textThe present paper reviews published experimental work focusing on condensation flow regimes, heat transfer and pressure drop in minichannels. New experimental data are available with high pressure (R410A), medium (R134a) and low pressure (R236ea) refrigerants in minichannels of different cross section geometry and with hydraulic diameters ranging from 0.4 to 3 mm.
Because of the influence of flow regimes on heat transfer and pressure drop, a literature review is presented to discuss flow regimes transitions. The available experimental frictional pressure gradients and heat transfer coefficients are compared with semi empirical and theoretical models developed for conventional channels and with models specifically created for minichannels.
Starting from the results of the comparison between experimental data and models, the paper will discuss and evaluate the opportunity for a new heat transfer model for condensation in minichannels; the new model attempts to take into account the effect of the entrainment rate of droplets from the liquid film
Circumferential profile of local heat transfer coefficient during condensation on enhanced tubes
No full textThe combined effect of vapour shear and condensate inundation during condensation on integral-fin tubes: a theoretical analysis
No full textCondensation heat transfer coefficients on enhanced tubes: experimental and theoretical analysis on vapour shear effects
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