Flow Boiling Heat Transfer of R134a in a Vertical Helically Coiled Tube

ABSTRACT

The boiling heat transfer characteristics of R134a in a helically coiled tube under high heat flux condition were investigated experimentally in present paper. The inner diameter and the coil diameter of the coil were 8 mm and 205 mm, respectively. Experiments were carried out with the heat flux in the range of 10–60 kW/m², pressure 0.8–1.1 MPa, mass flux 195–400 kg/(m² s) and vapor qualities 0.1–0.9. The results show that nucleate boiling plays a significant role even at high vapor quality. Inversely, the convective boiling was weakened in the high quality region under high heat flux conditions. The available boiling heat transfer coefficient correlations were compared with present experimental results. A new boiling heat transfer coefficient correlation for R134a in helically coiled tube was proposed based on the boiling mechanisms analysis. The new correlation was also proved to be applicable for high mass flux and low heat flux conditions.

Nomenclature
A	=	cross-sectional area, m2
Bo	=	boiling number, Bo = q /(G hfg)
cp	=	specific heat, J/(kg K)
d	=	tube diameter, m
D	=	curvature diameter, m
De	=	Dean number, Re(di/D)0.5
G	=	mass flux, kg/(m2 s)
h	=	heat transfer coefficient, W/(m2 K)
H	=	enthalpy, J/kg
Hfg	=	latent heat, J/kg
I	=	current, A
L	=	tube length, m
MAE	=	mean absolute error, $\mathrm{MAE}=\frac{1}{N}{\sum }_{j=1}^N|\frac{h_{\mathrm{pre},j}-h_{\exp ,j}}{h_{\exp ,j}}|$
MRE	=	mean relative error, $\mathrm{MRE}=\frac{1}{N}{\sum }_{j=1}^N\frac{h_{\mathrm{pre},j}-h_{\exp ,j}}{h_{\exp ,j}}$
N	=	the number of the experiment data
Nu	=	Nusselt number
P	=	pressure, MPa
Pr	=	Prandtl number
q	=	heat flux, W/m2
Q	=	heating power, W
Re	=	Reynolds number
T	=	temperature, K
U	=	voltage, V
x	=	vapor quality
Χtt	=	Martinelli parameter, ${\chi }_{tt}={\left(\frac{1-x}{x}\right)}^{0.9}{\left(\frac{{\rho }_g}{{\rho }_l}\right)}^{0.5}{\left(\frac{{\mu }_l}{{\mu }_g}\right)}^{0.1}$

Greek symbols

η	=	thermal efficiency
μ	=	viscosity, kg/(m s)
ρ	=	density, kg/m3

Subscripts

A	=	any location of the test tube
eq	=	equivalent
exp	=	experiment
E	=	electric heating
g	=	gas
i	=	inner
in	=	inlet
l	=	liquid
o	=	outer
out	=	outlet
P	=	preheater
pre	=	prediction
T	=	test section
tp	=	two-phase
sat	=	saturation
sp	=	single-phase
w	=	wall

Superscript

—

=

average

Additional information

Funding

This work was supported by China National Funds for Distinguished Young Scientists (51425603) and China National Key Research and Development Plan Project (2016YFB0600100).

Notes on contributors

Xiaojuan Niu

Xiaojuan Niu is a Ph.D. student at the State Key Laboratory of Multiphase Flow in Power Engineering at Xi'an Jiaotong University, Xi'an, China. She is working on boiling flow and heat transfer, critical heat flux, and flow patterns in the helically coiled tubes.

Huaijie Yuan

Huaijie Yuan is a Ph.D. student at the State Key Laboratory of Multiphase Flow in Power Engineering at Xi'an Jiaotong University, Xi'an, China. He is working on flow and heat transfer characteristics in micro-scale helically coiled tubes.

Chen Quan

Chen Quan is studying her master's degree at the State Key Laboratory of Multiphase Flow in Power Engineering at Xi'an Jiaotong University, Xi'an, China. She is working on flow patterns and pressure drop of boiling flow in helically coiled tubes.

Bofeng Bai

Bofeng Bai is a distinguished professor at the State Key Laboratory of Multiphase Flow in Power Engineering in Xi'an Jiaotong University. He received his Ph.D. degree in Power Engineering and Engineering Thermophysics from Xi'an Jiaotong University in 1999. He is the winner of the Distinguished Young Scientists of National Natural Science Foundation of China. His main research interests include the fundamentals of multiphase flow and heat transfer and their applications in petroleum engineering and power engineering, such as oil and gas recovery, phase-change heat exchangers, and propulsion and power systems

Liang Zhao

Liang Zhao is a professor at the State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University. He is also the vice dean of the School of Energy and Power Engineering, Xi'an Jiaotong University. He received his Ph.D. degree in Power Engineering and Engineering Thermophysics from Xi'an Jiaotong University in 2006. His research interests include multiphase flow and heat transfer, solar energy thermal utilization, thermal power generation, and hydrogen production.