Experimental and semi-analytical investigation of heat transfer in nucleate pool boiling by considering surface structuring methods

ABSTRACT

Pool boiling is the process in which the heating surface is submerged in a large body of stagnant liquid. In the present work, heat transfer in nucleate pool boiling is modeled both experimentally and semi-analytically with the consideration of surface structuring methods including wire electrical discharge machining (WEDM) and etching. The developed models consider variations of nucleation site density (NSD), contact angle and Prandtl number taking into account both pure and impure fluids. According to the results, by increasing wall superheat and contact angle, heat flux enhances. Also, rising Prandtl number results in heat flux enhancement. Comparing Sample 2 (structured by etching method), Sample 3 (structured by WEDM method), and Sample 4 (the surface with best performance) with Sample 1 (plain surface) indicates that the HTC is approximately 37%, 121%, and 177% higher than the plain surface, respectively. Comparison of the obtained results based on the semi-analytical model with existing experimental data and previous analytical models shows a good agreement.

KEYWORDS:

• Experimental model
• pool boiling
• nucleation sites density
• contact angle
• Prandtl number
• semi-analytical model

Nomenclature

Symbols

$A$	=	Surface area $\left(m^2\right)$
$Ar$	=	Archimedes number
$B$	=	Specific liquid constant
$C_P$	=	Specific heat capacity $\left(\frac{J}{kg\cdot K}\right)$
$D$	=	Diameter $\left(m\right)$
$f$	=	Bubble departure frequency $\left(\frac{1}{s}\right)$
$F$	=	Force $\left(N\right)$
$g$	=	Gravity acceleration $\left(\frac{m}{s^2}\right)$
$h$	=	Heat transfer coefficient $\left(\frac{W}{m^2\cdot K}\right)$
$Ja$	=	Jacob number
$k$	=	Thermal conductivity $\left(\frac{W}{m\cdot K}\right)$
$N_a$	=	Active nucleation site density $\left(\frac{site}{m^2}\right)$
$P$	=	Pressure $\left(Pa\right)$
$Pr$	=	Prandtl number
$q$	=	Heat flux $\left(\frac{W}{m^2}\right)$
$R_a$	=	Roughness $\left(\mu m\right)$
$T$	=	Temperature $\left(K\right)$
$t$	=	Time$\left(s\right)$

Greek symbols

$\alpha$	=	Thermal diffusivity $\left(\frac{m^2}{s}\right)$
$\delta$	=	Thickness of micro layer $\left(m\right)$
$\theta$	=	Contact angle
$\mu$	=	Dynamic viscosity $\left(Pa\cdot s\right)$
$\nu$	=	Momentum diffusivity $\left(\frac{m^2}{s}\right)$
$\rho$	=	Density$\left(\frac{kg}{m^3}\right)$
$\sigma$	=	Surface tension $\left(\frac{N}{m}\right)$

Subscripts

$bub$	=	Bubble
$d$	=	Dry
$g$	=	Growth
$me$	=	Microlayer evaporation
$nc$	=	Natural convection
$r$	=	Reformation
$tot$	=	Total
$w$	=	Waiting

Abbreviations

$CNC$	=	Computer Numerical Control
$CHF$	=	Critical Heat Flux
$DAQ$	=	Data Acquisition
$HTC$	=	Heat Transfer Coefficient
$IMN$	=	Interconnected Microchannel Nets
$NSD$	=	Nucleation Site Density
$PID$	=	Proportional Integral Derivative
$RTD$	=	Resistance Temperature Detector
$S$	=	Solid State Relay
$WEDM$	=	Wire Electric Discharge Machining