Comparison of pool boiling heat transfer for different tunnel-pore surfaces

Complex experimental investigations of boiling heat transfer on structured surfaces covered with perforated foil were performed. Experimental data were discussed for three kinds of enhanced surfaces: tunnel structures (TS), narrow tunnel structures (NTS) and mini-fins with the copper wire net (NTS-L). The experiments were carried out with water, ethanol, R-123 and FC-72 at atmospheric pressure. The TS and NTS surfaces were manufactured out of perforated copper foil (hole diameters: 0.3, 0.4, 0.5 mm) sintered with the mini-fins, formed on the vertical side of the 5 and 10 mm high rectangular main fins and horizontal inter-fin surface. The NTS-L surfaces were formed by mini-fins of 0.5 and 1 mm height uniformly spaced on the base surface. The wire mesh with an aperture of 0.32, 0.4 and 0.5 mm sintered with the fin tips formed a system of connected perpendicular horizontal tunnels. The tunnel width was 0.6 - 1.0 - 1.5 mm and the depth was 0.5 or 1.0 mm. The effects of the Bond number and dimensionless parameters for three kinds of enhanced structures on heat transfer ratio at nucleate pool boiling were examined.


Introduction
Boiling heat transfer enhancement is effective provided that specific surfaces are used (enhanced boiling surfaces). The studies conducted since the 1960s at numerous research centers have shown that the most efficient method of increasing the density of the heat flux and the value of the heat transfer coefficient is the use of coverings that are either porous or porous and capillary, the employment of enhanced surfaces or forming the channels, tunnels and hollows/cavities linked with the surface by a narrow "outlet". The following factors facilitate boiling heat transfer intensification: x artificially produced nucleation sites in the form of pores contribute to the increase in the nucleation site densities, x increasing the wetted surface by using the tunnels/channels and additionally extending the surface through the use of the fins/mini-fins/microfins, x film vaporization or vaporization from the menisci in the subsurface tunnels/hollows/cavities, x internal convection associated with the liquid flow in the tunnels/channels, forced by the capillary forces and the pumping action of the growing bubbles, x external convection associated with the growth, departure and movement of the vapor bubbles.
This article summarizes the results of the boiling heat transfer investigations conducted on three types of surfaces with subsurface tunnels. The measurement data for the four types of working fluid used in the studies were taken from [1,2,3,4].

TS surfaces
The specimens with the tunnel structures (TS) formed a square with sides of length 27 mm (w f ). The three main fins comprising the square were modified to include tunnels on the vertical surfaces and in the horizontal inter-fin spaces. The perforated copper foil was sintered to the machined surfaces (figure 1) creating a structure of combined U-shaped tunnels with the following parameters: x tunnel pitch: 2.0 -2.25 -2.5 mm (p tun ), x pore diameter: 0.3 -0.4 -0.5 mm (d p ), x pore pitch: 0.6 -0.8 -1.0 mm (p p =2d p ).

NTS surfaces
The main fins were thru-milled to form mini-fins with the width corresponding to the thickness of the base fins. Additionally, an array of combined tunnels was created, closed with the sintered perforated foil or the wire mesh. The parameters of the NTS specimens were as follows (figure 2): x fin number: 3, x fin height: 5 and 10 mm (h f ), x fin thickness: 5 mm (G f ), x inter-fin space width: 5 mm (s), x tunnel depth in inter-fin spaces: 1.0 mm (h tunH ), x tunnel width: 0.6 -1.0 -1.5 mm (w tun ), x tunnel pitch: 2.0 mm (p tun ), x pore diameter: 0.3 -0.4 -0.5 mm (d p ), x pore pitch: 0.6 -0.8 -1.0 mm (p p =2d p ).

NTS-L surfaces
These are the structural surfaces formed by sintering the woven copper wire mesh made of wire 0.14; 0.2; 0.32 mm in diameter to the mini-fin tips. The copper specimens, square in shape with a side of 26,5 mm, had 112 mini-fins and formed a system of tunnels intersecting at 90 degrees (figure 3).
Measurements were conducted also for NTS-L surfaces without a sintered wire mesh (plain mini-fins).
The following parameters are used ( figure 3): x mini-fin number: 112,

Experimental set-up
The experimental set-up designed for determining the boiling curves and heat transfer coefficients was composed of the following modules (

Bond number
The Bond number is the ratio of characteristic dimension to capillary length. For TS or NTS surfaces: and for NTS-L surface: where: d p -pore diameter, m a -mesh aperture dimension, m. The capillary length L characterizes potential for bubble departure and coalescence: where: U l saturated liquid density, kg/m 3 U v saturated vapor density, kg/m 3 V surface tension, N/m.
It is proportional to vapor bubble departure diameter.

Generalized results
The measurement data collected help to generalize the results in the form of the dependence of the selected surface heat transfer coefficient/smooth flat surface coefficient ratio on the Bond number and varying geometric parameters: x TS surface: