FIELD OF INVENTION
The present invention relates to a process of configurating microsized channels having
improved inclination and geometry for substantial enhancement of boiling heat transfer
over place surface. More particularly the invention relates to a technique for enhancing
the boiling heat transfer Co-efficient.
BACKGROUND OF INVENTION
The passive technique of increasing boiling heat transfer by providing different treated
surfaces has been investigated extensively for the last eight decades. Surfaces have been
designed to provide increased number of nucleation sites and to promote vapor removal
including replenishment of the liquid phase. These techniques have also been exploited
commercially due to its obvious advantages. Techniques to mass produce such surfaces
have been developed for both planner and cylindrical (internal and external) surfaces.
Some of the commonly used techniques for the enhancement of heat transfer coefficient
are as under:
(a) Roughened surface: The simplest method for enhancing the boiling heat transfer
coefficient is by roughening the test surface. Jacob and Fritz [1931] investigated the
effect of surface finish on nucleate boiling performance and concluded that roughness
can improve the boiling performance. Kurihari and Mayers [1960] boiled water and
various organic liquid on roughened copper

surface and obtained satisfactory result. Using high speed photography, Clark et
al. [1969] identified naturally occurring pits and scratches between 8 and 80 µm
widths as active boiling sites for pentane. After 80 µm width, the trend of
roughening the surface for the enhancement of boiling heat transfer begins in
full bloom. But the percentage increment of heat transfer coefficient is limited in
case of such type of boiling surfaces.
(b) Reentrant cavity: Griffith and Wallis [1960] first proposed that reentrant
cavities will be very efficient for enhancement of boiling heat transfer by their
theoretical analysis. Benjamin and Westwater [1961] were apparently the first to
construct a reentrant cavity and demonstrate its superior performance as a vapor
trap. Later in the mid-1960s, industrial researches started to achieve the goal of
an enhanced boiling surface for commercial application using reentrant cavities.
But the continual demand for incremental heat transfer coefficient leads the
researchers to search more enhancement techniques.
c) Porous coating: One of the first surfaces developed specifically to enhance
nucleate pool boiling by the porous sintered metallic coating by Thome [1995].
The 'particle spraying' technique was recently applied by You et al. [1992] to a
flat horizontal surface with a 0.3-3 µm AI2O3 particle. Later O'Connor and You
[1995] developed surfaces to enhance boiling heat transfer by painting it in order
to increase the number of active nucleation sites. This technique is also not
capable of handling large rate of heat transfer coefficient.

d) Extended surfaces: The use of extended surfaces to aid heat transfer has
been routinely practiced for decades. The standard method for achieving greater
heat transfer rates for a given heat exchanger volume in gas flows is to add
extended surfaces, normally in the form of fins. Current interest in fins for
enhancement of convective heat transfer includes developments of
discontinuities called louvers in the fin, and finning inside, as well as outside
tubes. A good amount of enhancement has been reported by these types of
surfaces.
e) Tunnel and pore type surfaces: Fujie et al. [1977], Saier et al. [1979],
Fujikake [1980] developed various reentrant tunnel and pore type surfaces and
patented them that are used for various heat transfer application. Recently
Ramaswamy et al. [2002] studied bubble growth using high speed photography
(1500 frames/sec) on micro-porous structures over subsurface tunnels immersed
in a pool of dielectric coolant (FC-72). They used wafer dicing and wet etching to
fabricate a net of interconnected micro-channels on silicon wafer. This is the
most efficient manner of surface modification developed so far for the
enhancement of boiling heat transfer.
Tunnel type of surface modification was first introduced by Kun and Czikk
[1969], who described a method to score a flat aluminum plate with closely
pitched parallel grooves (0.13 mm pitch) and 0.25 mm deep. The plate was
scored again perpendicular to the first set of grooves. This forms subsurface

cavities with restricted opening at the top which showed a better performance
for various liquids. In 1970 Webb developed bent fin surface and reported that
this type of surface is capable of reducing a large amount of degree of superheat
for a fixed heat flux.
Arshad and Thome [1983] conducted flow visualization from tunnel type surfaces
to understand the mechanism of boiling inside the channels. This study
confirmed that the primary mechanism of heat transfer inside the channels was
evaporation of the thin liquid menisci in sharp corners. The study also showed
that the vapor initiation was from one of the sharp corners and quickly spread to
occupy a large portion of the channel volume.
Later on Hahne et al. [1991] experimentally and theoretically studied the effect
of pool boiling heat transfer on tube with tunnel geometry. Hubner and Kunstler
[1997] developed trapezoid-shaped, T-shaped and Y-shaped tunnels on
sandblasted tubular surface and boiled fluorinated hydrocarbon on these tubes.
They reported that trapezoid-shaped tunnels produced much better performance
that plain tubes. T-shaped and Y-shaped tunnels are enhancing the boiling
performance further.
Webb and Pais [1992] developed five different horizontal tunnels over tubular
surfaces by wire electrode discharge machining process. Pool boiling data are
provided at 40°F(4°C) and 80°F (27°C) for five refrigerants boiling on a plain
tube, a GEW A K26 integral-fin tube, and three enhanced tubes. The pool boiling
coefficients for alternate refrigerants R-123 and R-l 34a are within 10% of the
values for R-11 and R-12, respectively, for all tubes, except the Turbo-B with R-
11/23.

Recently, Chien and Webb [1998] conducted high-speed visuali (100 frames/s)
of boiling in R-123 from a finned-tube which was covered with a thin sheet and
had pores at regular intervals (Fig. 1). The bubble departure diameter, frequency
of bubble formation and the bubble site density were measured. The bubble
growth data showed bubbles departing at a faster rate compared to those on a
plain surface. The study concluded that the bubble formation phenomenon was
thus different than that on a plain polished surface. The bubble data were used
to develop a semi-analytical model for the boiling process.
Ghiu et al.[2001] performed a visualization study of pool boiling from transparent
quartz structures of the reentrant form. The channel widths studied were 0.090
mm and 0.285 mm. The top of the structures was covered with a quartz plate
having the same overall dimensions (10 mm X 10 mm X 1mm ). Three boiling
regimes were identified: slug formation (in either top or bottom channels), slug
migration between the top and bottom channels and slug predominance (most of
channels vapor filled). Their study did not contain data for structures made of
higher thermal conductivity materials (copper, silicon).
Rajalu et al. [2004] carried out experimental investigation for pool boiling of
acetone, iso-propanol, ethanol and water, at atmospheric pressure, on single
horizontal reentrant cavity tubes of brass. The boiling heat transfer coefficient
has been found to increase with the rise in heat flux and it is lowered with the
reduction in cavity mouth size. They also developed a correlation to predict the
boiling heat transfer coefficient as a function of heat flux and cavity mouth size.

Thus, several techniques are already available for the enhancement of boiling
heat transfer coefficient. The most widely used passive techniques are those
involving the modification of surface geometry. Rough surface, treated surface,
extended surface are attractive because of its capability of producing high boiling
performance for lower temperature rise. Reentrant cavities and tunnels are also
popular nowadays. The main drawback with most of the prior art passive
techniques is the manufacturing intricacy. This restricts the use of these surfaces
for various components for thermal devices. There is no easy and simple
surfaces prepared and/or, proposed for the enhancement of boiling heat transfer
coefficient.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a process of
configurating micro sized channels having improved inclination and geometry for
substantial enhancement of boiling heat transfer over plane surface.
Another object of the present invention is to propose a process of configurating
micro sized channels having improved inclination and geometry for substantial
enhancement of boiling heat transfer over plane surface which are easy to
desigh, and can be manufactured by adapting simpler non-conventional
machining processes.
A yet another object of the present invention is to propose a process of
configurating micro sized channels having improved inclination and geometry for
substantial enhancement of boiling heat transfer over plane surface which
achieves high heat transfer coefficient using simple reentrant geometries below
the horizontal and inclined tunnels.

A further object of the present invention is to propose a process of configurating
micro sized channels having improved inclination and geometry for substantial
enhancement of boiling heat transfer over plane surface which eliminates typical-
manufacturing process like chemical etching, chemical milling and similar others,
to produce reentrant shape.
Another object of the present invention is to propose a process of configurating
micro sized channels having improved inclination and geometry for substantial
enhancement of boiling heat transfer over plane surface which suggests the
angle of inclination for the micro channels to maximize the rate of heat transfer.
A still further object of the invention is to propose a device for determining the
boiling characteristics of enhanced heat transfer surfaces.
SUMMARY OF INVENTION
Accordingly, there is provided a process of configurating microsized channels
having improved inclination and geometry for substantial enhancement of boiling
heat transfer over plane surface. The process includes Initially, surfaces with
tunnels of constant width (0.25mm), constant pitch (2 mm) and constant depth
(2mm) from the top surface, are developed. Variations in respect of the
inclination of the tunnel including the additional geometrical feature at the tunnel
base is selected. To differentiate between the different tunnel geometries, a
hexa-character symbolic specification has been adapted. For example;
unidirectional tunnel geometry with vertical tunnel (90° with horizontal), 2 mm
pitch, 2 mm depth, 0.25 mm width and Circular base geometry is denoted by U-
90-2-2-0.25-Cir. Surfaces are prepared by wire-electro discharge machining
(Wire-EDM or WEDM). Micro channel fabrication operation with one machining
pass is conducted using a low sparking energy by applying 70 to 100V, and a

maximum current of 6 to 10A with 0.08 to 1.02 µm ON and OFF time pulse
durations. Demineralized water of high insulation resistance is used as a
dielectric medium. Surface is cleaned thoroughly by alkaline cleaner, acid
cleaner and water mixture to remove oil or grease from it after preparation.
The invention further relates to a device for determining the boiling
characteristics of enhanced heat transfer surfaces.
The device is configured to determine the pool boiling heat transfer from
horizontal, smooth or structured surfaces. The main components of the device
are a boiling vessel , a heater assembly , a reflux condenser , a top cover ,
power supply and instrumentation . The cylindrical boiling vessel, stores the
liquid pool and forms a top part of the device. Liquid pool rests on a bakelite
plate which also facilitates a horizontal test plate to be in contact with the liquid
and resist the liquid to enter in the heater assembly . The horizontal test plate is
a part of the heater assembly and it protrudes inside the boiling vessel . A
copper plate is used as the test surface.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1- shows a typical Wire-EDM machine,
Figure 2(a)- shows a straight unidirectional surface geometry according to the
invention,
Figure 2(b)- shows a straight unidirectional surface geometry with circular base,
according to the invention,

Figure 2 (c )- shows a 60° inclined unidirectional surface geometry, according to
the invention,
Figure 2(d)- shows a 60° inclined unidirectional surface geometry with circular
base, according to the invention,
Figure 3(a)- shows a pictorial view of straight unidirectional surface geometry
according to the invention.
Figure 3(b)- shows a 60° inclined unidirectional surface geometry, according to
the invention
Figure 3(c) - shows a straight unidirectional surface geometry with circular base
according to the invention,
Figure 3(d)- shows a 60° inclined unidirectional surface geometry with circular
base according to the invention.
Figures 4(a) to 4 (d)- show cross-sectional view of surface geometry straight
tunnel with various degree of inclination according to the
invention.
Figures 5(a) to 5 (d)- show cross sectional view of surface geometry of straight
circular base tunnel with various inclinations according to
the invention.

Figure 6 (a)- a schematic view of a test device according to the invention.
Figure 6 (b)- detailed view of heater assembly of the device of Figure 6(a).
Figure 7- a graphical representation of boiling curves for enhanced and plane
surfaces according to the invention.
Figure 8- shows a boiling curve for various inclination angles for straight tunnel
according to the invention.
Figure 9- graphically shows the effect of angle of inclination in straight circular
base tubbel according to the invention.
Figure 10- shows an overall effect of tunnel inclination at 105°C according to the
invention.
Figure 11- shows a qualitative comparisons of the enhance surfaces according to
the invention.
DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS OF THE
INVENTION
Initially, surfaces with tunnels of constant width (0.25mm), constant pitch (2
mm) and constant depth (2mm) from the top surface, are developed. Variations
in respect of the inclination of the tunnel including the additional geometrical
feature at the tunnel base is selected. To differentiate between the different
tunnel geometries, a hexa-character symbolic specification has been adapted.
For example; unidirectional tunnel geometry with vertical tunnel (90° with
horizontal), 2 mm pitch, 2 mm depth, 0.25 mm width and Circular base

geometry is denoted by U-90-2-2-0.25-Cir as explained in Table 1. The details of
all these surfaces developed are given in Table 2.

Table 1 Significance of hexa-character symbol used for surface nomenclature
Surfaces are prepared by wire-electro discharge machining (Wire-EDM or
WEDM). WEDM is a machining method in which thermo electric energy (8000-
12000°C) is applied through a dielectric medium (14) between the tool electrode
(15) and an electrically conductive work piece (16). The material removal is
achieved by controlled erosion through a series of repetitive sparks between the
electrodes, that is,between the work-piece (16) and a wire (17). A schematic
view of the process is shown in Figure 1. Micro channel fabrication operation
with one machining pass is conducted using a low sparking energy by applying
70 to 100V, and a maximum current of 6 to 10A with 0.08 to 1.02 µm ON and
OFF time pulse durations. A copper electrode (15) coated with a zinc layer of
0.05 to 1.50 is used. The plate (16) is kept always connected to the positive
polarity and the electrode (15) to the negative polarity. Demineralized water of
high insulation resistance is used as a dielectric medium (14). Surface is cleaned
thoroughly by alkaline cleaner, acid cleaner and water mixture to remove oil or
grease from it after preparation. The geometry and the pictorial view of the
surfaces are shown in Figure 2 and 3 respectively. Experiments results show a
clear enhancement as depicted below:

To clearly bring out the effect of inclination, a second set of surfaces have been
configured where the angles are varied keeping the tunnel length same. Four
discrete angles-90° (Vertical, 60°, 45° and 30°) have been adapted. Additionally,
same tunnel geometries with circular pocket at the base have also been
developed. The surfaces are depicted in Figure 4 and 5.
Data on Table-2 show that the enhancement effect increases with the decrease
in angle of inclination. But after some limit (45°) boiling performance starts to
reduce with the further decrease in inclination.
The invention further proposes a device for determining the boiling
characteristics of enhanced surfaces. The device shown in Figure 6 ( a and b),
primarily constitutes a pool boiling device. The device is configured to determine
the pool boiling heat transfer from horizontal, smooth or structured surfaces. The
main components of the device are a boiling vessel (1), a heater assembly (2), a
reflux condenser (8), a top cover (10), power supply and instrumentation (12
and 13). The cylindrical boiling vessel (I), stores the liquid pool and forms a top
part of the device. Liquid pool rests on a bakelite plate which also facilitates a
horizontal test plate (3) to be in contact with the liquid and resist the liquid to
enter in the heater assembly (2). The horizontal test plate (3) is a part of the
heater assembly (2) and it protrudes inside the boiling vessel (1). A copper plate
is used as the test surface. The copper plate is press fitted to a copper block (4)
and which acts as the main part of the heater assembly (2). The detail of the
heater assembly is shown in Fig 1. a. The heater assembly (2) consists of four
cartridge heaters (5) for uniform heating of the cylinder. The side (6) and bottom
faces (7) of the cylinder are heavily insulated to facilitate unidirectional heat flow
only towards the top.

Fiber blanket insulation is provided around the copper cylinder while its bottom is
lagged by a ceramic insulation. The heating assembly (2) is placed inside a brass
casing. Teflon bush is disposed radially around the extended test plate (3) to
restrict the nucleation from the vertical surfaces. Liquid evaporated from the pool
is condensed in the reflux type condenser (8) and the condensate returns to the
pool due to gravity. This maintains a constant pool height over the boiling
surface during the operation. A heater coil (9) is provided for initial heating of
the pool and maintenance of the pool temperature during the test-run. The
Boiling vessel (1) is covered in the top by a bakelite plate which acts as a top
cover (10). Provisions are made for allowing auxiliary heater leads to come out of
the boiling vessel (1) and the generated vapor to enter into the condenser (8).
Power supply to the primary and secondary heaters is varied by controlling
variacs. To measure the average temperature of the test plate (3) , a copper
constantan (T type) sheathed thermocouple (shown in figure 6.b) is embedded
inside it through its bottom face. It is placed below the top surface of the test
plate (3) to get the accurate temperature reading.
Pool temperature is also measured by an insulated copper constantan
thermocouple placed above the test surface. Power input to the heaters and
voltage signals from the thermocouples are analyzed and stored using a data
acquisition means (12) and a personal computer (13). The heat flux is
determined by recording the voltage and current input to the heater assembly.
To prevent the leakage from the Copper-Teflon contact line and Bakelite-Teflon
contact line, high temperature thermal paste is used. This ensures leak-proof
joints up to a temperature of around 1000°C. A circular gasket is provided in the
glass bakelite junction to resist the vapor leakage from the boiling vessel (1). A
digital camera with a low frame rate is used to record the images of the growing
bubble and the boiling phenomena. Three leveling screws (11) are used to make
the test plate (3) perfectly horizontal.

After degassing the pool by vigorous boiling (with the help of the pool heater)
power is given to the heater. Power supply to the heater is increased in small
steps such that steady state conditions are achieved for a particular value of test
plate temperature. All the tests are carried-out under saturated pool condition. In
the device, the test surface is fitted to obtain the boiling performance of the
augmented surfaces. Results obtained from the tests are compared with the
performance of a plane surface to get qualitative information for enhanced
surfaces.
Firstly, a straight unidirectional parallel tunnel (U-90-3-2-0.4-N) is used for the
test, and the results obtained are shown in figure 7. From the figure, it can be
seen that U-90-3-2-0.4-N shows a definite enhancement over plane surface.
To obtain still better enhancement, a straight tunnel with circular base (U-90-3-
2-0.4-Cir) is then adapted. It serves both the effect of re-entrant cavity and
helps smooth removal of vapor. Using this surface, a boiling test has been done
and a substantial amount of enhancement has been obtained (Figure 7).
Performance of U-90-3-2-0.4-Cir is better than U-90-3-2-0.4-N. Actually the
circular base tunnel acts as a vapor pocket at the base and helps to maintain
continuous liquid menisci inside the tunnel circular base.
To study the effect of inclination, a second set of test is undertaken by adapting
4 different angle of inclination (90°, 60°, 45°, 30°). Both straight tunnels and
the inclined tunnels are adapted for this test.

Figure 8 presents the heat transfer characteristics from surfaces with straight
tunnels of different inclination. Compared to plane surface, all the surfaces have
high heat transfer rate as expected. All the surfaces having inclined tunnels
exhibit higher rate of heat transfer compared to the surface with vertical tunnels.
But interestingly, the rate of heat transfer monotonically increases when the
inclination angle is changed from 90° (vertical ) to 60° and further from 60° to
45°. However, when the inclination angle is changed to 30°, the rate of heat
transfer decreases.
Finally, to determine the effect of circular base at the end of the inclined tunnel
on the heat transfer characteristics, the boiling curves obtained from such
surfaces are plotted and shown in Figure 9. In this case also the rate of heat
transfer increases as the inclination angle decreases from 90° to 60° to 45°. But
thereafter it decreases as the inclination angle is changed from 45° to 30°. This
corroborates the earlier observation presented in figure 8.
In Figure 10 a comparative view for the heat flux with respect to angle of
inclination is plotted at a fixed degree of superheat to get a realistic view of the
effect of angle of inclination.
From the test results, it may be concluded that continuous longitudinal tunnels
can be another option for augmentation. Two geometrical configurations of the
tunnel geometry offer definite advantage.

• Firstly, inclined tunnels show better performance compared to vertical
tunnels. But after a certain degree of inclination, the heat transfer
coefficient decreases.
• Secondly, reentrant geometry like circular pocket at the end of the tunnel
base enhances the boiling heat transfer further. It may further be
mentioned that such geometry needs the least manufacturing effort.
A qualitative comparison among all the surfaces prepared along with plane
surface has been shown in Figure 11 to get an overview of the characteristics of
the enhanced surface.

WE CLAIM
1. A process of configurating microsized channels having inclination of 40°-50° and
for enhancement of boiling heat transfer over plane surface, comprising the steps
of;
developing surfaces with tunnels;
configurating variations in respect of the inclination of the tunnel comprising
additional features by adapting a hexa-character symbolic specification;
cleaning the developed surface by alkaline cleaner, acid cleaner and water
mixture;
characterized in that,
the tunnels of the developed surfaces are of constant: width, constant pitch and
constant depth when surfaces are prepared by wire-electro discharge machining
while demineralized water of high insulation resistance works as dielectric medium
(14) wherein micro channel fabrication operation with one machining pass is
conducted with a low sparking energy.
2. The process as claimed in claim 1, wherein the micro-channel fabrication is
conducted by applying 70 to 100V, and a maximum current of 6 to 10 amp with
0.08 to 1.02 µm ON and OFF time pulse duration.

3. The process as clamed in claim 1, wherein the surfaces with tunnels of constant
width of 0.25mm, constant pitch of 2mm, and constant depth of 2mm from the
top surface are prepared by applying thermo electric energy (8000-12000°C)
comprising application of a dielectric medium (14) between a tool electrode (15)
and an electrically conductive work piece (16).
4. The process as claimed in claim 3, wherein the material removal is carried out by
controlled erosion through a series of repetitive sparks between the electrodes.

ABSTRACT

A PROCESS OF CONFIGURATING MICROSIZED CHANNELS HAVING IMPROVED
INCLINATION AND FOR ENHANCEMENT OF BOILING HEAT TRANSFER OVER
PLANE SURFACE
A process of configurating microsized channels having inclination of 40°-50° for
enhancement of boiling heat transfer over plane surface, comprising the steps of
developing surfaces with tunnels, configurating variations in respect of the inclination of
the tunnel comprising additional geometrical features by adapting a hexa-character
symbolic specification and cleaning the developed surface by alkaline cleaner, acid
cleaner and water mixture when the tunnels of the developed surfaces are of constant
width, constant pitch and constant depth when surfaces are prepared by wire-electro
discharge machining while demineralized water of high insulation resistance works as
dielectric medium wherein micro channel fabrication operation with one machining pass is
conducted with a low sparking energy.