Flow Boiling Heat Transfer and Dryout Characteristics of R600a in a Vertical Minichannel

Abstract

Refrigerant-related environmental concerns forced legislative bodies to phase out some types of refrigerants, namely, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) and in the near future European legislation will be affecting hydrofluorocarbons (HFCs) as well. Natural refrigerants such as hydrocarbons can thus be expected to be more common as refrigerants in the future. Experimental findings on flow boiling heat transfer and dryout characteristics of isobutane (R600a) in a uniformly heated, vertical, stainless-steel test section (1.60 mm inside diameter and 245 mm heated length) are reported in this article. The experiments were conducted at two saturation pressures corresponding to the temperatures of 27 and 32°C, with five mass fluxes in the range 50–350 kg/m²-s and at outlet vapor qualities up to dryout conditions. Analysis showed that heat transfer was primarily controlled by the applied heat flux with insignificant effect of mass flux and vapor quality. The dryout heat flux increased with increasing mass flux; however, no significant effect of varying saturation temperature was observed. The experimental results (for heat transfer and dryout) were compared with different macro and microscale correlations from the literature.

NOMENCLATURE

Ac	=	cross-sectional area (m2)
Ah	=	heated area (m2)
Bd	=	Bond number
Bo	=	Boiling No.
CFC	=	chlorofluorocarbon
Cp	=	specific heat capacity (J/kg-K)
Co	=	Confinement number
din	=	inside diameter of test section (m)
E	=	nucleate boiling enhancement factor (—)
G	=	mass flux (kg/m2-s)
HCFC	=	hydrochlorofluorocarbon
HFC	=	hydrofluorocarbon
hlg	=	latent heat of vaporization (kJ/kg)
I	=	current (A)
k	=	thermal conductivity (W/mk)
lh	=	heated length (m)
M	=	molecular weight (kg/kmol)
	=	mass flow rate (kg/s)
PR	=	reduced pressure (—)
q′′	=	heat flux (kW/m2)
Re	=	Reynolds number
S	=	nucleate boiling suppression factor (—)
V	=	voltage (V)
We	=	Weber number
x	=	vapor quality (—)
z	=	axial position (m)
zo	=	single phase length (m)

Greek Symbols

ρ	=	density (kg/m3)
μ	=	viscosity (Pa-s)
σ	=	surface tension (N/m)

Subscripts

Cooper	=	Cooper pool boiling correlation
D-B	=	Dittus–Boelter
Exit	=	at outlet of the test section
g	=	gas phase
in	=	refers to inner surface/inlet of test section
l	=	liquid phase
lo	=	liquid only
nb	=	nucleate boiling

Additional information

Notes on contributors

Zahid Anwar

Zahid Anwar is a Ph.D. student at the Department of Energy Technology, Division of Applied Thermodynamics and Refrigeration, KTH, Sweden. He is working on experimental analysis of phase-change heat transfer in narrow channels with low-GWP refrigerants. He completed his master's degree in Sustainable Energy Engineering program from KTH in 2011.

Björn Palm

Björn Palm is professor and head of the Department of Energy Technology, Royal Institute of Technology, KTH, Sweden. He is also head of the Divison of Applied Thermodynamics and Refrigeration at KTH. His research interests are in the field of heat transfer related to refrigeration and heat pump systems. He completed his Ph.D. from KTH in 1991, and his thesis was entitled “Enhancement of Boiling Heat Transfer by Aid of Perforated Metal Foils.”

Rahmatollah Khodabandeh

Rahmatollah Khodabandeh is an associate professor at the Department of Energy Technology, Royal Institute of Technology, KTH, Sweden. His research focuses on topics of cooling of electronics, heat transfer in small channels, and thermosyphons. He completed his Ph.D. degree from KTH in 2004, and his thesis was entitled “Heat Transfer and Pressure Drop in a Thermosyphon Loop for Cooling of Electronic Components.”