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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
Experimental study on condensation heat transfer inside a single circular minichannel
No full textThe measurement of the condensation heat transfer coefficient inside micro- and minichannels is still somewhat elusive due to the difficult task of getting accurate values of the heat transfer coefficients during the condensation process, particularly when studied within single minichannels. The present paper reports local heat transfer coefficients obtained from the measurement of the local heat flux and the direct measurement of the saturation and wall temperatures during condensation of R134a and R32 within a single circular 0.96 mm diameter minichannel. Except for the lowest mass velocity, the test results do not show significant discrepancy from the trends expected for macroscale tubes
Effect of cross sectional shape during condensation in a single square minichannel
No full textThe objective of this work is to present new condensation heat transfer coefficients measured inside a
single square cross section minichannel, having a 1.18 mm side length, and compare them to the ones
previously measured in a circular minichannel. Tests have been performed with R134a at 40°C saturation
temperature, at mass velocity ranging between 200 and 800 kg m-2 s-1. As compared to the heat transfer
coefficients measured in the circular cross section channel, for the same hydraulic diameter, in the square
minichannel the authors find a heat transfer enhancement at the lowest value of mass velocity, which
must be due to the effect of surface tension. No heat transfer coefficient increase has been found at
the highest values of the mass velocity where condensation is shear stress dominated
Experimental study of condensation inside a horizontal single square minichannel
No full textThis work is aimed at presenting experimental heat transfer coefficients measured during condensation inside a single square cross section minichannel, having a 1.18 mm side length. The experimental heat transfer coefficients are compared to the ones previously obtained in a circular minitube. This subject is particularly interesting since most of the mini and microchannels used in practical applications have non circular cross sections. The test section used in the present work is obtained from a thick wall copper tube which is machined to draw a complex passage for the water; its geometry has been studied with the aim of increasing the external heat transfer area and thus decreasing the external heat transfer resistance. This experimental technique allows to measure directly the temperature in the tube wall and in the water channel. The heat flux is determined from the temperature profile of the coolant in the measuring sector. The wall temperature is measured by means of thermocouples embedded in the copper tube, while the saturation temperature is obtained from the saturation pressure measured at the inlet and outlet of the measuring sector. On the whole, more than seventy thermocouples have been placed in the 23 cm long measuring section. Tests have been performed with R134a at 40°C saturation temperature, at mass velocities ranging between 200 and 800 kg m−2s−1. As compared to the heat transfer coefficients measured in a circular minichannel, in the square minichannel the authors find a heat transfer enhancement at the lowest values of mass velocity; this must be due to the effect of the surface tension. No heat transfer coefficient increase has been found at the highest values of the mass velocity where condensation is shear stress dominated.</jats:p
Frictional pressure drop during vapour-liquid flow in minichannels: Modelling and experimental evaluation
No full textCondensation in minichannels is widely used in air-cooled condensers for the automotive and air-conditioning
industry, in heat pipes and other applications for system thermal control. The knowledge of pressure
drops in such small channels is important in order to optimize heat transfer surfaces. This paper
presents a model for calculation of the frictional pressure gradient during condensation or adiabatic
liquid–gas flow inside minichannels with different surface roughness. In order to account for the effects
of surface roughness, new experimental frictional pressure gradient data associated to single-phase flow
and adiabatic two-phase flow of R134a inside a single horizontal mini tube with rough wall has been
used in the modelling. It is a Friedel (1979) [Friedel, L., 1979. Improved friction pressure drop correlations
for horizontal and vertical two-phase pipe flow. In: Proceedings of the European Two-Phase Flow Group
Meeting, Ispra, Paper E2] based model and it takes into account mass velocity, vapor quality, fluid properties,
reduced pressure, tube diameter, entrainment ratio and surface roughness. With respect to the
flow pattern prediction capability, it has been built for shear dominated flow regimes inside pipes, thus,
annular, annular-mist and mist flow are here predicted. However, the suggested procedure is extended to
the intermittent flow in minichannels and it is also applied with success to horizontal macro tubes
Pressure drop during two-phase flow of R134a and R32 in a single minichannel
No full textCondensation in minichannels is widely used in air-cooled condensers for the automotive
and air-conditioning industry, heat pipes, and compact heat exchangers. The knowledge
of pressure drops in such small channels is important in order to optimize heat transfer
surfaces. Most of the available experimental work refers to measurements obtained
within multiport smooth extruded tubes and deal with the average values over the number
of parallel channels. In this context, the present authors have set up a new test apparatus
for heat transfer and fluid flow studies in single minichannels. This paper presents new
experimental frictional pressure gradient data, relative to single-phase flow and adiabatic
two-phase flow of R134a and R32 inside a single horizontal minitube, with a 0.96
mm inner diameter and with not-negligible surface roughness. The new all-liquid and
all-vapor data are successfully compared against predictions of single-phase flow models.
Also the two-phase flow data are compared against a model previously developed by
the present authors for adiabatic flow or flow during condensation of halogenated refrigerants
inside smooth minichannels. Surface roughness effects on the liquid-vapor flow are
discussed. In this respect, the friction factor in the proposed model is modified, in order
to take into consideration also effects due to wall roughness
Effect of minichannel cross sectional shape in condensation
No full textThe objective of this work is to present new condensation heat transfer coefficients measured inside a single square cross section minichannel, having a 1.18 m side length, and compare them to the ones measured in a circular minichannel. This subject is particularly interesting since most of the mini and microchannels used in practical applications have non circular cross sections. The test section used in the present work is obtained from a thick wall copper tube which is machined to draw a complex passage for the coolant. This experimental technique allows to measure directly the temperature in the tube wall and in the water channel. The heat flux is determined from the temperature profile of the coolant in the measuring sector. The experimental technique is checked in single phase runs which represent a severe testing condition due to the low values of heat flux. Tests have been performed with R134a at 40°C saturation temperature, at mass velocities ranging between 200 and 800 kg m−2s−1. As compared to the heat transfer coefficients measured in a circular minichannel, in the square minichannel the authors find that heat transfer is enhanced at the lowest values of mass velocity; this must be due to the effect of the surface tension. No heat transfer coefficient increase has been found at the highest values of the mass velocity where condensation seems to be shear stress dominated.</jats:p
Forced convection condensation inside minichannels: guidelines for a new predictive procedure
No full textCondensation inside minichannels finds many applications, such as in heatpipes for electronic machinery or for spacecraft thermal control, or in automotive condensers. It has
also been suggested for air-conditioning and refrigeration applications, in order to design
compact heat exchangers, to reduce refrigerant charge, to use natural refrigerants, to
improve cycle efficiency.
Because of the influence of flow regimes on heat transfer andp ressure drop, a literature
review is presented to discuss flow regimes transitions. The available experimental heat
transfercoefficients were already compared by the present authors in previous papers with
semiempirical and theoretical models developed for conventional channels and with models
specifically developed for minichannels. The assessmentof the results pointed out the need
for new design methods.
Starting from this standpoint, a new procedure is reported in the present paper which takes
into account the effect of the entrainment rate of droplets from the liquid film
Heat transfer coefficient during condensation of a high pressure refrigerant insida a circular minichannel
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