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Effects of geometry and fluid properties during condensation in minichannels: experiments and simulations

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Abstract

The present paper aims at investigating the condensation process inside minichannels, at low mass fluxes, where bigger discrepancies from conventional channels can be expected. At high mass flux, the condensation in minichannels is expected to be shear stress dominated. Therefore, models originally developed for conventional channels could still do a good job in predicting the heat transfer coefficient. When the mass flow rate decreases, the condensation process in minichannels starts to display differences with the same process in macro-channels. With the purpose of investigating condensation at these operating conditions, new experimental data are here reported and compared with data already published in the literature. In particular, heat transfer coefficients have been measured during R134a and R1234ze(E) condensation inside circular and square cross section minichannels at mass flux ranging between 65 and 200 kg m−2 s−1. These new data are compared with those of R32, R717, R290, R152a to show the effect of channel shape and fluid properties and to assess the applicability of correlations developed for macroscale condensation. For this purpose, a new criterion based on the Weber number is presented to decide when the macroscale condensation correlation can be applied. The present experimental data are also compared against three-dimensional Volume of Fluid (VOF) simulations of condensation in minichannels with circular and square cross section. This comparison allows to get an insight into the process and evaluate the main heat transfer mechanisms.

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Abbreviations

Bo [−]:

Bond number, Bo = [(ρL- ρV)gDh2]/σ

Bocr [−]:

Critical Bond number, Bocr = [ρL/(ρL- ρV)-π/4]−1

D h [m]:

Hydraulic diameter

e p [%]:

Percentage deviation, ep = 100·(HTCCALC- HTCEXP)/ HTCEXP

e R [%]:

Average deviation, eR = (1/Np)∑ep

g [m s−2]:

Acceleration due to gravity

G [kg m−2 s−1]:

Mass flux

HTC [W m−2 K−1]:

Cross-sectional average heat transfer coefficient

J V [−]:

Dimensionless gas velocity, JV = xG/[gDhρV(ρL-ρV)]0.5

k [m2 s−2]:

Turbulent kinetic energy

N p [−]:

Number of data points

p [bar]:

Pressure

R [m]:

Channel radius

T [K]:

Temperature

t [m]:

Average liquid film thickness

t + [−]:

Dimensionless average liquid film thickness, t+ = t/y*

U VS [m s−1]:

Superficial velocity of vapour phase, UVS = xG/ρV

U LS [m s−1]:

Superficial velocity of liquid phase, ULS = (1-x)G/ρL

We [−]:

Weber number, We = ρV(UVS-ULS)2Dh/σ

x [−]:

Vapor quality

X tt [−]:

Martinelli parameter, Xtt = (μL/μV)0.1(ρV/ ρL)0.5[(1-x)/x]0.9

y [m]:

y - coordinate

y* [m]:

Length scale, y* = μL(τW/ ρL)-0.5/ ρL

α t + [−]:

Dimensionless turbulent eddy diffusivity for heat

ΔT [K]:

Temperature difference

λ [W m−1 K−1]:

Thermal conductivity

μ [Pa s]:

Dynamic viscosity

νt + [−]:

Dimensionless turbulent eddy diffusivity for momentum

ρ [kg m−3]:

Density

σ [N m−1]:

Surface tension

σ N [%]:

Standard deviation, σN = {[∑(ep-eR)2]/(Np-1)}0.5

τW [Pa]:

Wall shear stress

ω [s−1]:

Specific dissipation rate of turbulent kinetic energy

CALC:

Calculated

EXP:

Experimental

L :

Liquid

S :

Saturation

T :

Transition

V :

Vapour

W :

Wall

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Acknowledgments

The authors acknowledge the financial support of MIUR (Ministero dell’Istruzione, dell’Università e della Ricerca) through the program PRIN 2015 (Grant Number 2015M8S2PA) and the support of the European Space Agency through the MAP Condensation program ENCOM-3 (AO 2004-096).

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Authors and Affiliations

  1. Dipartimento di Ingegneria Industriale, Università degli Studi di Padova, Via Venezia 1, I-35131, Padova, Italy

    Paolo Toninelli, Stefano Bortolin, Marco Azzolin & Davide Del Col

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  1. Paolo Toninelli
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  2. Stefano Bortolin
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  3. Marco Azzolin
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  4. Davide Del Col
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Correspondence to Davide Del Col.

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Toninelli, P., Bortolin, S., Azzolin, M. et al. Effects of geometry and fluid properties during condensation in minichannels: experiments and simulations. Heat Mass Transfer 55, 41–57 (2019). https://doi.org/10.1007/s00231-017-2180-7

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  • DOI: https://doi.org/10.1007/s00231-017-2180-7

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