1. A thermosiphon loop for high heat flux removal using flow boiling of
ethanol in OMM with taper
Sanskar S. Panse, Satish G. Kandlikar ⇑
Mechanical Engineering, Rochester Institute of Technology, Rochester, NY, USA
a r t i c l e i n f o
Article history:
Received 15 July 2016
Received in revised form 4 September 2016
Accepted 6 September 2016
Available online xxxx
Keywords:
OMM
Tapered manifolds
Gravity-driven flow
Thermosiphon loop
Flow boiling
Electronics cooling
a b s t r a c t
Open Microchannel Manifold (OMM) with taper has been effective in enhancing heat transfer perfor-
mance during flow boiling with low pressure drop. This makes it very attractive in low pressure drop sys-
tems like a thermosiphon loop. A gravity-driven flow boiling system was used to generate the
performance data using ethanol at low flow rates. Based on the pressure drop and heat transfer data, a
two-phase thermosiphon loop with a small ethanol head of 0.2 m was developed and tested with
OMM configuration. A maximum heat flux of 136 W/cm2
was recorded at a wall superheat of 42 °C
resulting in a highest heat transfer coefficient of 34,100 W/m2
K with 6% taper manifold. The ther-
mosiphon loop provided a maximum flow rate of approximately 35 mL/min. Pressure drop data showed
stable thermosiphon operation and a very low pressure drop of only 4 kPa near CHF. The heat transfer
performance was independent of the orientation of the test section as the horizontal and the vertical
upflow configurations yielding similar values of heat transfer.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
High heat flux dissipation using single-phase liquid has
received considerable attention since the pioneering work of Tuck-
erman and Pease [1] in 1980s. Although this technique has proved
effective in several cooling applications, the high pressure drop still
remains a concern. Two-phase heat transfer utilizes latent heat of
vaporization of liquid and is capable of dissipating high heat fluxes
while maintaining lower surface temperatures. This makes it a
promising candidate for cooling high heat dissipation systems in
earth-based and microgravity environments alike. Two-phase
cooling with microchannels is attractive in various applications
including servers, turbine blades, solar arrays, boilers and onboard
electronics in aircrafts and satellites, to name a few. Jet impinge-
ment, vapor chamber, heat pipes and thermosiphon loops have
been extensively studied for electronics cooling applications. This
work explores the performance of a compact two-phase ther-
mosiphon loop featuring Open Microchannel Manifold (OMM)
with taper geometry for high heat flux removal in electronics
applications such as data centers. An exhaustive review highlight-
ing some of the major contributions in this field is not included
here, but only a very brief review is presented to highlight the need
for developing a high heat flux dissipating thermosiphon loop.
Jet Impingement has been investigated with single-phase and
two-phase flow by a number of investigators, e.g. [2,3]. It employs
a single or multiple jets of working fluid normal to the heated sur-
face through a nozzle. Heat transfer is maximum in the impinge-
ment zone and decreases away from the nozzle, leading to non-
uniformity in surface temperature [2]. Qiu et al. [2] extensively
reviewed the various jet impingement techniques and the factor
affecting it performance like jet parameters (impact velocity,
impact distance, jet diameter, subcooling etc.) and surface param-
eters (surface conditions, surface aging etc.). They reported that jet
parameters largely affected boiling performance. Surface parame-
ters also played a crucial role in influencing boiling performance
with micro and nano porous structures enhancing boiling heat
transfer. Browne et al. [3] experimentally studied boiling perfor-
mance with R134a in a submerged microjet array. They varied
the liquid subcooling as 10, 20 and 30 °C and jet velocity as 4, 7
and 10 m/s and found that the presence of non-condensable gases
(NCG) enhanced the heat transfer and achieved a maximum heat
flux of 590 W/cm2
with heat transfer coefficient improving to
110,000 W/m2
K from 75,000 W/m2
K. Similar dependence on
NCG was observed by Zhou et al. [4]. Despite the advantages
of jet impingement cooling to provide high heat transfer
performance, its dependence on high coolant flow rate requires
http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020
0017-9310/Ó 2016 Elsevier Ltd. All rights reserved.
⇑ Corresponding author at: Mechanical Engineering, Rochester Institute of
Technology, 76 Lomb Memorial Dr., Rochester, NY 14623, USA.
E-mail addresses: sp2506@rit.edu (S.S. Panse), sgkeme@rit.edu (S.G. Kandlikar).
International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer
journal homepage: www.elsevier.com/locate/ijhmt
Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

2. substantial pumping power. The high velocity also causes erosion
of the heat transfer surfaces.
Heat pipe and vapor chambers are compact passive electronic
cooling devices with no moving parts, where working fluid is
transported from the condenser to the evaporator through micro
size wick structures by capillary forces. The performance of heat
pipes and vapor chambers is limited by their size and device
dimensions [5]. They are preferred where device compactness is
desired and the condensing surfaces can be placed in close proxim-
ity of the evaporator surface. Nishikawara and Nagano [6] carried
out a parametric study and investigated the effect of gap between
wick and the evaporator, filling ratio, temperature of heat sink and
working fluid properties on performance of heat pipe. R134a, ace-
tone and ethanol were chosen as working fluids in their study. At
the maximum applied heat load of 40 W they achieved a heat flux
of 4 W/cm2
with ethanol, with optimal wick-evaporator gap height
of 20 lm.
Ji et al. [7] proposed a novel Extended Vapor Chamber (EVC)
with an extended condenser design. Along with grooved network
for better vapor circulation, capillary holes were machined within
the solid fin surface. Copper foam pipe, used as wicking structures
were then inserted into these grooves. With this arrangement they
were able to achieve high thermal performance with minimum
thermal resistance of 0.03 K/W and maximum heat flux of
445 W/cm2
without reaching dryout. A number of recent works
have shown the high heat flux dissipation capability of vapor
chambers with heat spreaders. These systems are very effective
when the condenser can be placed in close proximity as mentioned
earlier.
Flow boiling in small diameter conduits for fluid flow, like mini
and microchannels, have shown potential in enhancing two-phase
heat transfer [1,8,9]. However, these systems have an inherent dis-
advantage of imposing high pressure drops for fluid flow, thus
demanding additional external pumping requirements. Moreover,
instabilities during flow boiling due to bubble nucleation and
growth lead to flow reversals and pressure fluctuations, thereby,
affecting its performance considerably [10,11].
Mukherjee and Kandlikar [12] numerically studied the growth
of a bubble inside a microchannel and suggested channels with
increasing cross-sectional area like diverging and stepped
microchannel configuration to promote unidirectional fluid flow
and reduced flow instabilities. Lu and Pan [13] employed diverging
microchannels in conjunction with 25 equally spaced artificial
nucleation sites along the length of microchannels. They found
diverging microchannels instrumental in stabilizing the flow, with
artificial nucleation sites enhancing heat transfer performance and
minimizing wall temperature. Miner et al. [14] numerically studied
the effect of microchannel with expanding base on flow boiling
performance. Their model used CHF as a measure of performance
enhancement in microchannels. They predicted CHF augmentation
higher than 600 W/cm2
through analysis of expanding microchan-
nels. In a corresponding experimental study, Miner et al. [15] stud-
ied pressure drop in parallel microchannels with an expanding
base at various expansion angles. They found considerable reduc-
tion in pressure drop from over 100 kPa to 40 kPa at the highest
mass flow rate at an expansion angle of 2°.
A similar approach was undertaken by Balasubramanian et al.
[16]. They tested the flow boiling performance of deionized water
using stepped microchannels. Stepped microchannels were fabri-
cated by reducing the fin height by 400 lm over a certain length,
at predetermined locations from the inlet. A very high heat flux
around 400 W/cm2
was achieved at wall superheat of 15 °C with
one of their stepped microchannel geometries.
Kandlikar et al. [17] introduced Open Microchannel Manifold
(OMM) geometry by providing a gap over the microchannels for
greater fluid flow area. Also, a taper was provided in the manifold
with increasing flow area downstream. The OMM with taper
geometry was found to be instrumental in enhancing heat transfer
performance while maintaining a very low pressure drop [18,19].
In light of the superior performance achieved using OMM with
taper, Buchling and Kandlikar [20] explored the performance of
ethanol in pumpless, gravity-driven flow boiling system. Flow boil-
ing performance of ethanol was tested at flow rates 40, 60 and
80 mL/min with plain and microchannel chips. A heat flux of
217 W/cm2
was dissipated at system pressure drop of just 9 kPa.
Based on heat transfer and pressure drop performance, they high-
lighted the opportunity in developing a compact cooling device
like a thermosiphon loop with OMM geometry.
Similar to heat pipes and vapor chambers, thermosiphon loops
are passive two-phase electronic cooling devices. Unlike heat pipes
and vapor chambers, where fluid flow from the condenser to the
evaporator is achieved through capillary force action, a ther-
mosiphon loop accomplishes this through gravity and liquid–vapor
density difference. Thermosiphon loops have been studied exten-
sively by researchers for high heat dissipation capacity without
the requirement for external pumping power. Performance of a
thermosiphon is influenced by factors like, (i) input power, (ii) fill-
ing ratio, (iii) aspect ratio (evaporator to condenser length), (iv)
system pressure, and (v) working fluid. Effect of the first three fac-
tors on heat transfer characteristics were studied by Noie [21].
They varied the heat load from 100 W to 900 W and found that
maximum heat transfer for different aspect ratios takes place at
different fill ratios. While optimal fill ratio for aspect ratio of 7.45
was 90%, it was 60% and 30% for aspect ratios 11.8 and 9.8 respec-
tively. They recorded a maximum heat transfer coefficient of
3500 W/m2
K. Khodabandeh and Palm [22] studied the effect of
system pressure on heat transfer performance of thermosiphon
loop. They found strong dependence of thermal performance on
system pressure with enhancement in heat transfer coefficient
with increase in pressure. By increasing reduced pressure from
0.02 to 0.3 bar, they found performance improvement from
280 kW/m2
(28 W/cm2
) at 22 °C wall superheat to 300 kW/m2
(30 W/cm2
) at 7.5 °C with isobutane. Khodabandeh [23] varied
the driving liquid head and found that there was very little differ-
ence in heat transfer coefficients at the tested heat fluxes with heat
transfer coefficient varying slightly between 20 and 25 kW/m2
K.
Nomenclature
q00
heat flux, W/cm2
Up error uncertainty
Dp pressure drop, kPa
H liquid head, mm
q liquid density, kg/m3
g gravitational acceleration, m/s2
Ts surface temperature, °C
kCu thermal conductivity of copper, W/m K
dT temperature difference, °C
dx vertical separation between thermocouples, mm
h heat transfer coefficient, W/m2
K
DTsat degree of wall superheat, °C
2 S.S. Panse, S.G. Kandlikar / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx
Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

3. Khodabandeh [24] found the pressure drop in the riser tube from
the evaporator to the condenser to be nearly an order of magnitude
higher than that across the evaporator, with a maximum value of
nearly 5 kPa at the highest heat flux.
It is seen that the thermosiphons offer a pumpless system, but
their performance is rather limited. This work builds up upon the
research carried out in [20] that indicates the possibility of incor-
porating an OMM design with taper in a thermosiphon loop to dis-
sipate high heat fluxes. Towards that objective, additional heat
transfer and pressure drop data for the OMM geometry with taper
is generated at low flow rates in gravity-driven flow boiling sys-
tem. Based on this heat transfer and pressure drop data, a compact
two-phase thermosiphon loop was developed and tested to the
critical heat flux limit.
2. Experimental setup
2.1. Gravity-driven flow boiling system for low flow rate performance
testing of OMM with taper
The gravity-driven flow boiling setup developed by Buchling
and Kandlikar [20] shown Fig. 1 was used in the first part of the
present work . It consists of two fluid reservoirs, at a vertical sep-
aration of 2.68 m. This provided a maximum available pressure
head of 20 kPa with ethanol. Performance of this setup was tested
at low flow rates of 10, 20 and 40 mL/min. It consisted of two main
sections: (1) main fluid line, and (2) degassing line. The main fluid
line featured two fluid reservoirs, rope preheaters to achieve the
required subcooling, the test section featuring open microchan-
nels, tapered manifold and copper heater block, and a fluid con-
denser. The degassing line was integrated with the system near
valve V-03 and aided in the removal of non-condensable gases
from the system. The setup and test procedure are described in
more detail in [20].
2.2. Two-phase thermosiphon loop
The thermosiphon loop shown in Fig. 2 was used in the second
part of the work. It consists of an evaporator section featuring the
open microchannel heat sink with tapered manifold, copper heater
block, and the condenser section featuring fluid reservoir and con-
densing copper coils. The evaporator and the condenser are verti-
cally separated by 0.15 m (150 mm) to provide the required
driving head for ethanol flow. The required driving head is com-
puted based on the pressure drop and heat transfer results
acquired with gravity-driven flow boiling setup at low flow rates.
The setup consists of vertical tubes of identical cross-sections
namely, downcomer and riser at the evaporator inlet and outlet
respectively for fluid transport from the condenser to the evapora-
tor and back. The riser is longer than the downcomer and extends
further into the condensing chamber, above the liquid level in the
chamber. This ensures one-way fluid flow from the condenser to
the evaporator through the downcomer.
The condensing chamber which serves as liquid ethanol reser-
voir consists of water-cooled copper condensing coils. The liq-
uid–vapor mixture from the riser condenses back and settles
back into the reservoir, thus maintaining the desired overall fluid
volume in the system. Two K-type thermocouples are placed to
record the fluid temperature at the evaporator outlet and the liquid
ethanol temperature inside the condenser, while a pressure gauge
is also installed within the chamber. Atmospheric pressure is
maintained inside the chamber over the entire range of the tests
by adjusting the coolant flow rate in the condenser. The liquid
height H within the condensing chamber is varied to investigate
the effect of ethanol head on the flow boiling performance.
A separate degassing loop is integrated in the system, attached
to the condensing chamber. Degassing of ethanol is carried out by
heating it to saturation temperature within the evaporator. Liquid-
vapor ethanol mixture along with non-condensable gases rises into
the chamber through the riser. The non-condensable gases are sep-
arated from the liquid–vapor mixture at the condenser, where con-
densed ethanol vapors settle back in the chamber as liquid, while
non-condensable gases continue downstream. These gases are
removed from the system via the vacuum pump, which is set to
gauge pressure below 1.3 kPa, through the liquid–vapor separator
chamber. This procedure is allowed to continue for a period of 1
hour once the fluid saturation temperature is reached at the evap-
orator outlet, after which, the fluid is assumed to be sufficiently
degassed.
2.2.1. Experimental procedure
Before the commencement of a boiling test in the thermosiphon
loop, a predetermined volume of ethanol was filled in the condens-
ing chamber metered with the help of a syringe pump. Tests were
run at ethanol fill volumes of 90 mL and 150 mL. These fill volumes
corresponded to an ethanol head of 15 mm and 25 mm respec-
tively within the chamber in addition to 150 mm of available head
between the evaporator and the condenser. Once the fluid was suf-
ficiently degassed, the main heater unit connected to the copper
heater block was engaged. Ball valve V-01 was opened to allow
the fluid flow from the condenser to the evaporator through the
downcomer. The main heater unit was operated at a step incre-
ment of 5 V (corresponding approximately to 10 W) to the point
where the surface temperature reached 120 °C, at which point,
the power to the main heater is switched off. Owing to the absence
of a CHF mitigation loop that was present in the earlier setup, all
thermosiphon tests were concluded before reaching the CHF to
avoid damage to the setup due to temperature overshoot.
3. Test section
The test section shown in Fig. 3(a) presents a tapered manifold
and open microchannel heat sink assembly. The microchannel heat
sink featured in this study is selected as the best performing
microchannel geometry, referred to as MC1, in Buchling and Kan-
dlikar [20]. The OMM with taper configuration addresses the issues
of flow instability and low CHF incurred in flow boiling in
microchannels by allowing easy bubble growth and escape and
replenishment of liquid to prevent channel dry-out. Ceramic blocks
insulate the heat sink to minimize heat loss to the surrounding.
Three thermocouple holes were drilled below the chip surface for
heat flux calculation. The topmost hole was drilled 1.5 mm below
chip surface with offset of 3 mm between adjacent holes.
The manifold was fabricated from lexan polysulphone, which
has a glass transition temperature of 140 °C and was polished to
provide clear visualization with a high speed camera. Four types
of manifold tapers, one uniform and three tapered manifolds, were
employed in the study. The microchannel chip had channel depth,
channel width and fin width of 200 lm each and was machined on
top of the heater block, as shown in Fig. 3(b). A silicon gasket
placed between the chip and the manifold, apart from sealing the
test section, ensured only 10 Â 10 mm chip area was exposed to
the fluid and a uniform manifold gap height of 127 lm is main-
tained throughout. The heater block accommodated four ports
for 4 Â 400 W cartridge heaters drilled near the bottom. A heat loss
analysis of the heater block was carried out using ANSYS Fluent in
[25]. The central region of the copper chip surface in contact with
the working fluid was perfectly insulated and heat loss from edge
of the chip surface was computed as a function of temperature
S.S. Panse, S.G. Kandlikar / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx 3
Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

4. difference between the chip and ambient. The heat loss was found
to be around 3 W for a typical test.
4. Data reduction
Heat flux through the heater block is calculated using the Four-
ier’s law for 1-D conduction,
q00
¼ ÀkCudT=dx: ð1Þ
The temperature gradient in Eq. (1) is approximated using
Taylor’s backward difference,
dT
dx
¼
3T1 À 4T2 þ T3
2Dx
ð2Þ
where T1, T2 and T3 are the temperature reading from the top,
middle and bottom thermocouples respectively and Dx is the
separation between adjacent thermocouples, which is 3 mm.
Fig. 1. Gravity-driven flow boiling setup.
4 S.S. Panse, S.G. Kandlikar / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx
Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

5. The surface temperature is extrapolated using top thermocou-
ple and the heat flux, accounting for thermal resistance of copper
and the separation between the top thermocouple and chip surface
x1 as,
Ts ¼ T1 À q00
ðx1=kCuÞ ð3Þ
Heat transfer coefficient is calculated taking into account the
chip surface temperature, heat flux and saturation temperature
of the fluid as,
h ¼ q00
=DTsat ð4Þ
5. Uncertainty analysis
A comprehensive uncertainty analysis was undertaken to com-
pute uncertainties associated with each measured and computed
quantity. Standard expression for error propagation for any prop-
erty p as a function of an independent variable r, over n samples
is given by,
Up ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Xn
i¼1
@p
@ri
Uri
v
u
u
t ð5Þ
Fig. 2. Thermosiphon loop.
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Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

6. All uncertainty measurements fell within the 95% confidence
interval. The total uncertainty values in the results are shown as
error bars in all the figures.
The maximum uncertainty associated with heat flux was below
10%, not exceeding 11 W/cm2
at higher heat fluxes. Uncertainty in
heat transfer coefficient measurement was up to 10%, below
7000 W/m2
K.
6. High-speed visualization
High speed visualizations were carried out using Photron
FASTCAM 1024PCI to study bubble growth and flow patterns
associated with ethanol. The camera has an ability to capture
high-speed videos at 100,000 fps, however, visualizations were
carried out at frames rates 3000–6000 fps for most of the tests in
conjunction with a fiber optic light source for illumination.
7. Results
7.1. Gravity-driven flow boiling system testing
The gravity-driven flow boiling system was tested to evaluate
its heat transfer and pressure drop performance at low flow rates
of 10, 20 and 40 mL/min. All tests were carried out at atmospheric
pressure, giving ethanol a saturation temperature of 78.3 °C. All
tests were run over a range of heat fluxes through CHF. Flow boil-
ing performance was evaluated as a function of heat flux, wall
superheat and pressure drop. Performance plots showing heat
transfer and pressure drop data and their relevance in the develop-
ment of thermosiphon loop is given in this section.
7.1.1. Heat transfer performance
Fig. 4 shows boiling curves for all tested manifolds at 10, 20 and
40 mL/min flow rate over the microchannel chip. In Fig. 4
(a) and (b), a performance improvement is seen as taper is
increased from uniform to 4% with a slight dip in the performance
with 6% taper at flow rates of 10 and 20 mL/min. However, a con-
sistent rise in heat flux is seen from uniform to 6% taper at higher
flow rate of 40 mL/min. A highest performance of 158 W/cm2
at
wall superheat of 44 °C was achieved with 6% taper at this flow
rate. At lower flow rates, a decent heat transfer performance was
obtained, with 4% taper exhibiting the best performance. The high-
est flux of 115 W/cm2
at 48.3 °C wall superheat was recorded at
20 mL/min.
7.1.2. Pressure drop performance
Fig. 5 shows pressure drop as a function of heat flux at 10, 20
and 40 mL/min. Pressure drop increased gradually with increase
in heat flux, with higher taper angle providing lower pressure drop
as expected.
Lowest pressure drop of only 0.92 kPa was obtained with 6%
taper. The pressure drop was comfortably below 6 kPa over the
entire range of heat fluxes tested at such low flow rates.
7.1.3. Ethanol head as a function of heat flux
Based on the pressure drop data obtained through experiments,
the gravity head required to drive ethanol through the system
without requiring a pump was computed for each heat flux.
Fig. 6(a) and (b) show the required head at flow rates of 10, 20
and 40 mL/min and the corresponding heat fluxes. It acts as a pre-
dictor into what performance to expect if a certain ethanol head is
applied.
An ethanol head around 0.5 m is enough to dissipate heat flux
around 150 W/cm2
in open microchannel and tapered manifold
geometry at flow rate of 40 mL/min. The required head increases
with heat flux. For a lower flow rate system, lower heads are
enough to obtain a moderately high heat transfer performance.
From Fig. 6, a head of 0.2 m is capable of achieving a heat flux over
80 W/cm2
without any external pumping power in this
configuration.
The driving head was calculated using the relation for static liq-
uid head,
H ¼ Dp=qg ð6Þ
7.2. Thermosiphon loop testing
To investigate the effect of taper on the boiling performance of
thermosiphon loop, 4% and 6% taper manifolds were tested with
the same microchannel heat sink with ethanol. Since 4% and 6%
tapers showed the highest performance with low pressure drop
in the gravity-driven flow boiling system, their performance was
investigated in the thermosiphon loop where ethanol is fed at a
constant gravity head.
Parametric study was undertaken to study the effect of ethanol
fill volume (FV) in the reservoir on the overall performance. FV was
Fig. 3. Schematic of (a) tapered manifold block, (b) heater block and microchannel
heat sink assembly.
6 S.S. Panse, S.G. Kandlikar / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx
Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

7. varied as 90 mL and 150 mL which correspond to an ethanol head
of 15 mm and 25 mm inside the condensing chamber (see Table 1).
This equaled to total ethanol head of 165 mm and 175 mm respec-
tively between the evaporator and the condenser. All tests were
stopped before CHF due to absence of a CHF control mechanism
in place.
7.2.1. Heat transfer performance
The microchannel heat sink was tested in tapered manifold
geometry by varying the ethanol fill volume as 90 mL and
150 mL in the thermosiphon loop. The results in Fig. 7 show
negligible effect of ethanol fill volume on the heat dissipation
performance. The best performance with this configuration
was recorded with 6% taper manifold, having a heat flux of
136 W/cm2
at 42.7 °C wall superheat with FV = 90 mL.
7.2.2. Pressure drop performance
Fig. 8(a) and (b) compare fluctuations in pressure drop in the
test section over a period of 10 s at heat fluxes of 30 W/cm2
and
Fig. 4. Plots showing boiling curves for all manifold tapers at (a) 10 mL/min, (b)
20 mL/min, (c) 40 mL/min flow rates.
Fig. 5. Plots showing pressure drop performance for all manifold tapers at (a)
10 mL/min, (b) 20 mL/min, (c) 40 mL/min flow rate.
S.S. Panse, S.G. Kandlikar / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx 7
Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

8. 100 W/cm2
in 4% and 6% tapers. Large variations in pressure drop
with periodic negative spikes signify flow fluctuations and rever-
sals, especially at lower heat fluxes. These negative spikes are
absent at the higher heat flux signifying stable thermosiphon oper-
ation with increase in heat flux with little to no flow reversals. The
flow was most stable with 6% taper manifold and the pressure drop
was consistently below 3.5 kPa. It is clear that the system goes
through an unstable region as the heat flux is increased. This
should be mitigated in a practical system. A simple solution could
be obtained by introducing a one-way valve prior to the evaporator
inlet in the loop.
8. Discussions
A detailed analysis of the results in the previous section is pre-
sented here.
8.1. Effect of taper
The manifold taper was effective in enhancing the heat transfer
performance in the gravity-driven flow boiling system. From Fig. 4
(a) and (b), tapered manifolds performed better than the uniform,
with performance improving with taper gradient from uniform to
2% to 4%. However, performance lowered slightly with further
increase in taper to 6%. This may be due to the low flow rates
employed which allow for greater vapor accumulation in the mani-
fold gap height due to low mass flux and thus weak liquid inertia.
This contributed to flow fluctuations and poor heat transfer. How-
ever, manifold taper was instrumental in maintaining a low pressure
drop due to the pressure recovery effect of tapered manifolds. Pres-
sure drop was lowest with 6% taper at 0.92 kPa and 1.32 kPa at
10 mL/min and 20 mL/min from Fig. 5(a) and (b) respectively.
Fig. 9(a) and (b) show the effect of taper on heat transfer
performance of the thermosiphon loop at 90 mL fill volume. The
Fig. 6. Plots showing ethanol head as a function of heat flux for (a) 4% taper, (b) 6% taper manifolds.
Table 1
Thermosiphon loop test matrix.
Manifold taper 4% 6%
Fill volume (mL)/total head (mm) 90/165 150/175 90/165 150/175
Fig. 7. Plots showing boiling curves for (a) 4% taper, (b) 6% taper manifolds.
8 S.S. Panse, S.G. Kandlikar / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx
Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

9. performance improved with increase in taper gradient from 4% to
6%. Higher taper gradient was also seen beneficial in mitigating
flow reversals, as seen in Fig. 8. The ability of thermosiphon loop
to self-sustain the flow and inherent capability of OMM to promote
easy vapor removal and liquid replenishment to nucleation sites
proved advantageous in enhancing heat transfer and pressure drop
performance.
8.2. Effect of ethanol fill volume
Fig. 7 shows little to no change in the dissipation performance
with increase in the fill volume. However, performance slightly
diminished in case of 6% taper. However, it is too early to make
any judgments based on the limited data available and thus, fur-
ther testing with different fill volumes is expected to give a clear
picture of its effect on performance.
8.3. Gravity-driven flow boiling versus thermosiphon loop
Fig. 10(a) and (b) compare the boiling performance in 4% and 6%
manifolds in thermosiphon loop with that of gravity-driven flow
boiling system at 20 and 40 mL/min in the same configuration.
For 4% manifold, the boiling
performance of the thermosiphon loop completely overlaps
with that of flow boiling at 20 mL/min. While, for 6% manifold, it
prominently improved over that at 20 mL/min and fell slightly
short of performance curve for 40 mL/min. Due to the absence of
a flow meter in the thermosiphon loop (to avoid additional pres-
sure drop), this superimposition of data gives a good understand-
ing on its behavior in tapered manifold geometry. This data is
then used to interpolate the fluid flow rate through the system.
An approximate flow rate in the system and the corresponding exit
quality is tabulated in Table 2 at the highest heat flux. The exit
vapor quality was computed thermodynamically based on the
energy balance on the fluid at the inlet and outlet of the test sec-
tion as,
x ¼
1
hfg
q00
A
m
À cpDTsub
ð7Þ
These plots also highlight the pressure recovery effect (due to
increasing flow area in the flow direction) in higher manifold
tapers in enhancing fluid flow rate through a system, where fluid
is fed from a fixed head. Thus, in addition to enhancing the heat
transfer while keeping the pressure drop to a minimum, taper is
seen to be beneficial in self-regulation of fluid flow rate which
serves to increase the heat transfer coefficient.
Fig. 8. Plots comparing pressure drop fluctuations at low and high heat fluxes in (a) 4% taper; (b) 6% taper manifolds.
Fig. 9. Plots showing effect of taper on (a) boiling curve, (b) heat transfer coefficient at 90 mL fill volume.
S.S. Panse, S.G. Kandlikar / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx 9
Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

10. Fig. 10(a) and (b) also highlight the ability of the thermosiphon
loop to provide a superior performance than the gravity-driven
flow boiling system even with a smaller available head. Fig. 6(a)
and (b) predicted a performance close to 100 W/cm2
for a head
of 0.2 m. However, results have shown a higher performance. This
may be due to two factors, (a) The reliance of thermosiphon on liq-
uid–vapor buoyancy difference for fluid flow which allows easy
escape of vapor through the riser tube, thus allowing better replen-
ishment of liquid to the heated surface. The vapor escape is also
assisted by the taper present on the manifolds. Flow through the
gravity-driven flow boiling system was driven solely by gravity
and the performance primarily depended on the liquid inertia,
i.e. fluid flow rate. At lower flow rates, vapor accumulation on
the heated surface is seen as the main cause for diminished perfor-
mance along with low heat transfer coefficients associated with
them. (b) The varying degree of subcooling. The liquid temperature
in the reservoir and thus, the liquid subcooling was seen to
increase with increase in heat flux and wall temperature. The fluid
temperature in the downcomer varied between 35 °C and 65 °C
over the range of the entire test. Higher inlet fluid temperature
promotes better heat transfer coefficient and thus better thermal
performance [26]. In the gravity-driven flow boiling system, inlet
liquid temperature was maintained at 55 °C.
8.4. Effect of flow orientation
An additional study was carried out to observe the effect of ori-
entation on the performance of the thermosiphon loop by allowing
the fluid to flow upwards on the heat sink surface. Fig. 11
(a) and (b) compare the heat transfer performance at horizontal
(0°) and vertical upflow (90°) orientations in 4% manifold at
90 mL fill volume. The plots show similar performance at these ori-
entations. The 6% taper manifold wasn’t tested as the primary goal
was to investigate effect of orientation change on performance and
not to carry out a comparative study of manifold geometries.
High speed images shown in Fig. 12 show downstream expan-
sion of nucleating bubbles. The bubble marked in the dark circle
expands as it moves downstream before exiting the test section.
It is seen to rise above the channel height into the gap provided
by the manifold taper. The upward bubble growth into the mani-
fold space allows for improved liquid supply to the nucleating site,
Fig. 10. Plots comparing boiling performances of thermosiphon loop and gravity-driven flow boiling system at 20 and 40 mL/min in, (a) 4% taper, (b) 6% taper manifolds.
Table 2
Estimated flow rate and exit quality.
Taper Flow rate (mL/min) Exit quality (x)
4% 20 0.31
6% 35 0.22
Fig. 11. Plots comparing (a) boiling curve, (b) heat transfer coefficient at horizontal (0°) and vertical upflow (90°) orientations for 4% taper manifold.
10 S.S. Panse, S.G. Kandlikar / International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx
Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

11. as liquid can fill the bottom of the channels as the bubbles continue
to grow. The marked and its adjacent bubbles show a pointed front
as it moves downstream which shows their natural tendency to
expand downstream in the tapered manifold. Due to the low fluid
inertia at low flow rates, the bubbles rise and expand solely by the
action of buoyancy forces in this orientation. The ability of OMM to
provide separate liquid–vapor pathways complimented by upward
flow orientations is a promising pathway for future research into
enhancing performance of thermosiphon loop.
9. Conclusions
Open Microchannel Manifold (OMM) with taper was tested in a
gravity-driven flow boiling system at low flow rates of 10, 20 and
40 mL/min with ethanol. Based on the heat transfer and pressure
drop performance plots acquired through the testing, a ther-
mosiphon loop was developed and tested with a small driving
head. Following conclusions were drawn through the experimental
results:
a. In the gravity-driven flow boiling system, OMM showed
good performance at low flow rate, with a maximum heat
dissipation of 157.6 W/cm2
at 43.9 °C wall superheat, while
maintaining a pressure drop of just 3.15 kPa at 40 mL/min.
Even at the lowest flow rate of 10 mL/min, a heat flux of
90 W/cm2
at a wall superheat of 41.5 °C was obtained, with
a very low pressure drop of 1.12 kPa. This ability of OMM to
provide superior heat transfer while maintaining a low pres-
sure drop performance makes it a good choice in developing
a compact, pumpless flow boiling system with a small driv-
ing head for electronics cooling applications.
b. The capability of OMM with tapered manifold to promote
easy vapor escape and liquid replenishment to the heated
surface is well established through literature to provide high
heat transfer performance with increase in manifold taper
downstream. However, at low flow rates of 10 and 20 mL/
min, higher taper gradient proved detrimental due to low
liquid inertia, which eventually led to excess vapor accumu-
lation in the manifold recess. This was the leading cause of
flow and pressure fluctuations in case of 6% taper manifold,
which consistently performed lower than the 4% manifold at
these flow rates.
c. A self-sustaining, two-phase thermosiphon loop was devel-
oped based on the performance trends from testing of
gravity-driven flow boiling system and was tested in
OMM. A maximum head flux of 136 W/cm2
at 41 °C was
obtained with 6% taper. The performance was seen to
increase with increase in taper gradient from 4% to 6%. The
pressure recovery effect of tapered manifold resulted in a
remarkably low pressure drop. The maximum pressure drop
was consistently below 4 kPa. Thus, the implementation of
OMM with higher taper gradients can be a promising path-
way for developing more compact cooling modules and
enhance heat transfer in future works.
d. The pressure recovery effect of the tapered manifold geom-
etry led to enhancement of fluid flow rate at higher heat
fluxes, thereby improving the heat transfer performance.
The heat transfer performance of the thermosiphon loop
was seen to exceed that of the gravity-driven flow boiling
system even with a smaller available head. Flow instabilities
were observed at low heat fluxes. These can be easily
avoided by placing a one-way valve that is commonly
employed in many thermosiphon loops to prevent large
pressure excursions.
e. This present study of OMM in a thermosiphon loop confirms
its ability to provide high heat dissipation even with a small
available head. This configuration can be further explored by
varying and optimizing parameters like microchannel geom-
etry, taper gradient, manifold gap height, liquid driving head
and flow orientation for further investigation of heat transfer
performance while making the system more compact.
Acknowledgments
This work was conducted in the Thermal Analysis, Microfluidics
and Fuel Cell Laboratory at Rochester Institute of Technology, NY.
This work was partly supported by the National Science Founda-
tion under award no. CBET-1236062.
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taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020

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Please cite this article in press as: S.S. Panse, S.G. Kandlikar, A thermosiphon loop for high heat flux removal using flow boiling of ethanol in OMM with
taper, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.020