The Effect of Transient Power Hotspots on the Heat Transfer Coefficient during Flow Boiling Inside Single Microscale Channels

ABSTRACT

This paper presents flow boiling experimental results concerning the transient behavior of the heat transfer coefficient (HTC) of a hotspot region undergoing through dynamic heat flux variations. The experimental data were obtained for R134a flowing inside a horizontal 1.1-mm internal diameter stainless-steel tube at a saturation temperature of 31°C, mass velocities of 400 and 600 kg m⁻² s⁻¹, local vapor quality of 8 and 40% and time-averaged heat flux of 80 and 120 kW m⁻². The hotspot region undergoes over-sinusoidal, square, and sawtooth heat pulsation at frequencies of 0.5, 1, and 2 Hz and half amplitudes of 20 and 40 kW m⁻². A fast response type-K thermocouple of 13-µm wire diameter was used to capture the wall temperature fluctuations during the tests. Parametric analyses are performed and the effects of heat pulses severity, waveform, frequency, and amplitude on the local HTC and wall superheat temperature are assessed. Also, the influences of mass velocity and vapor quality on the transient behavior of the HTC are analyzed.

Nomenclature

CPU	=	Central processing unit
D Diameter	=	Diameter m
F Frequency	=	Frequency Hz  or s− 1
f Friction factor	=	Friction factor −
G Mass flux	=	Mass flux kg m− 2 s− 1
HTC Heat transfer coefficient	=	Heat transfer coefficient   W m− 2 K− 1
h Enthalpy	=	Enthalpy kJ kg− 1
I Electrical current	=	Electrical current A
k Thermal conductivity	=	Thermal conductivity W m− 1 K− 1
L Length	=	Length m
ΔL Discrete element length	=	Discrete element length m
$\dot{m}$ Mass flow rate	=	Mass flow rate   kg s− 1
PV	=	Photovoltaic
$\dot{P}$ Electrical power	=	Electrical power W
p Pressure	=	Pressure Pa
Δp Pressure drop	=	Pressure drop Pa
q″ Heat flux	=	Heat flux W m− 2
$\overline{q^{\text{'}\text{'}}}$ Time-averaged heat flux	=	Time-averaged heat flux W m− 2
Δq″ Heat flux half-amplitude	=	Heat flux half-amplitude W m− 2
$\stackrel{\dots }{q}$ Volumetric heat generation	=	Volumetric heat generation W m− 3
T Temperature	=	Temperature °C
ΔT Temperature superheat	=	Temperature superheat K
V Voltage drop	=	Voltage drop V
x Vapor quality	=	Vapor quality −

Greek symbols

α Heat transfer coefficient	=	Heat transfer coefficient W m− 2 K− 1
ρ Density	=	Density kg m− 3
σ Relative heat losses	=	Relative heat losses %

Subscripts

1∅	=	Single-phase
2∅	=	Two-phase
bg	=	Background
diff	=	Difference
ext	=	External
f	=	Fluid
hpot	=	Hotspot
i	=	Discrete element index
in	=	Inlet
int	=	Internal
max	=	Maximum
out	=	Outlet
pre	=	Preheater
sat	=	Saturation
w	=	Wall

Acknowledgments

The technical support given to this investigation by Mr. José Roberto Bogni and Mr. Jorge Nicolau dos Santos is appreciated and recognized.

Additional information

Funding

The authors gratefully acknowledge the financial support given by CAPES (Coordination for the Improvement of Higher Level Personal, Brazil) and CNPq (National Council of Technological and Scientific Development, Brazil) through Grant Number 303852/2013-5. Fundação de Amparo à Pesquisa do Estado de São Paulo, (2016/09509-1).

Notes on contributors

Gustavo Matana Aguiar

Gustavo M. Aguiar has a Bachelor (2015) and a Master (2017) degree in Mechanical Engineering from the University of São Paulo. His main research interests include heat transfer and fluid flow in microscales and modeling and simulation of thermal systems.

Gherhardt Ribatski

Gherhardt Ribatski is Associate Professor and Coordinator of Mechanical Engineering Graduate Program at São Carlos School of Engineering, University of São Paulo (USP), Brazil. He received his B.S., M.S., and Doctoral degrees in Mechanical Engineering from the University of São Paulo. He is President of the Brazilian Society of Mechanical Sciences and Engineering, member of Assembly of World Conferences on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Virtual Institute of Two-Phase Flow and Heat Transfer, Scientific Council of the International Centre for Heat and Mass Transfer (ICHMT), and corresponding member of The ICeM NEWSLETTER/The Japanese Society for Multiphase Flow. He has presented eight keynote lectures and taken part in the scientific committee of several International Conferences. He has over 70 refereed journal publications, 6 book chapters, and over 120 refereed papers in conferences.