Proceedings of the 4th International Forum on Heat Transfer, IFHT2016
November 2-4, 2016, Sendai, Japan

IFHT2016-1986

HEAT TRANSFER CONTROL BY LIGHT IRRADIATION TO LOWREYNOLDS NUMBER FLOWS USING A PHOTOSENSITIVE MICELLARSOLUTION
Takeshi ENYA1, Reiko KURIYAMA1*, Kazuya TATSUMI1, Kazuyoshi NAKABE1
1

Kyoto University, Kyoto daigaku-katsura, Nishikyo-ku, Kyoto 615-8540, Japan

ABSTRACT
For the improvement in performance of compact heat exchangers, the authors are aiming at controlling heattransfer rates in milli-scale flows, with photo-rheological fluids (PRFs), viscoelasticity of which can be tuned
by light. In our previous study, ortho-methoxy cinnamic acid (OMCA) known as a photo-sensitive molecule
was added to a wormlike micellar aqueous solution. OMCA irreversibly isomerizes from trans-OMCA to cisOMCA upon exposure to UV light. This photo-isomerization alters the molecular packing at the micellarinterface, shortens wormlike micelles, and structurally weakens the viscoelasticity of the solution. This study
demonstrates light irradiation to PRFs flowing in a milli-scale serpentine channel to change mean heat transfer
coefficient and pressure loss penalty. We installed a light irradiation part upstream of the heat transfer part,and performed a heat transfer experiment and pressure-drop measurement in the downstream region. As aresult, both mean heat transfer coefficient and pressure loss penalty were decreased, which is supposed to becaused by weaker viscoelasticity of the solution that may significantly influence flow structure, as OMCA
undergoes a photo-isomerization. This result suggests the possibility that heat transfer performance can becontrolled by light.

KEY WORDS: Viscoelasticity, Photo-rheological fluids, Micellar solution, Heat transfer characteristics, Low
Reynolds number flow

1. INTRODUCTION
In recent years, the heat rate generated from a unit area of various electric devices increases with downsizing
and integration of electronic circuits, which causes problems in reliable operation of these devices. Thus forthe development of a high-performance small heat exchanger, various methods to improve working fluids andflow channel geometry have been extensively studied. Since Reynolds number decreases in small ducts, a flow
becomes laminar, and heat transfer performance decreases in comparison with the turbulence where a fluid ismixed by unsteady vortices. One of the methods to solve this problem is using viscoelastic fluids as working
fluids. In a viscoelastic flow, unsteady secondary flows are generated even at low Reynolds number regime
where a laminar flow is normally formed in the case using Newtonian fluids. Tatsumi et al. [1, 2]
experimentally and numerically investigated the flow structure and heat transfer characteristics of viscoelastic
flows in a millimeter-scale serpentine channel. They showed that both heat transfer coefficients and wall
friction factors were increased by utilizing a polymer solution as a working fluid, since the viscoelasticity of
the fluid induces flow perturbations and generates unsteady secondary flows. This implies that more pumping
energy to supply working fluids is required for the enhanced heat transfer in viscoelastic fluids than in
Newtonian fluids.
In order to overcome this trade-off problem related to the viscoelasticity, the authors are trying to develop an
energy saving method of heat transfer control by changing degree of viscoelasticity of a working fluid. Photorheological fluids (PRFs), whose rheological properties can be controlled by light, have gathered attention and*Corresponding Author: kuriyama@me.kyoto-u.ac.jp

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IFHT2016-1986
been researched in some engineering fields [3, 4], to date. The authors’ research group has been also
investigating the characteristics of PRF flows and showed that the viscoelasticity of a photo-sensitive wormlike
micellar solution, which is one of the PRFs, was decreased by light irradiation. They also showed that by using
this pre-irradiated solution as a working fluid, heat transfer rate and pressure drop were decreased incomparison with the case using Newtonian fluids [5, 6].
In consideration of the implementation to an actual installation of PRFs to heat transfer devices, this study
performs light irradiation to a PRF flowing in a channel upstream of the heat transfer target channel and insitu switching of viscoelasticity of a photo-sensitive micellar solution. Then, a heat transfer coefficient and apressure loss in the target channel are measured in order to evaluate the influence of the light irradiation on theheat transfer performance of PRFs.

2. Working Fluids
A working fluid used in the present study was cetyl-tri-methyl-ammonium bromide (CTAB) and sodium
salicylate (NaSal) mixture aqueous solutions in which ortho-methoxy cinnamic acid (OMCA) is added. The
concentrations of CTAB/NaSal/OMCA solution (CNO solution) were set to be 1.0wt% (CTAB), 0.2wt%(NaSal), and 0.2wt% (OMCA). OMCA is a photo-sensitive molecule which has trans- and cis-arrangementsof substituent. Figure 1 (a) and (b) shows the schematic behaviour of OMCA photo-isomerization and molar
absorption coefficient,  [L/(mol･cm)] of trans-OMCA and cis-OMCA, respectively. Figure 1 (b) represents
trans-OMCA irreversibly undergoes a photo-isomerization to cis-OMCA upon exposure to UV light, which is
attributed to the fact that  of trans-OMCA is larger than  of cis-OMCA at wavelength in the range of 250350nm. In this study, trans-OMCA is added to an aqueous mixture of CTAB and NaSal which are known for
inducing the formation of wormlike micelles [7]. When trans-OMCA molecules are taken in micelle-aggregateaqueous solution, they shorten distance between CTAB molecules. Then, long wormlike micelles are formed,
the micelles become entwined with each other, and the development of micelle network structure is promoted.
The aqueous solution eventually shows viscoelasticity which can be similarly seen in polymer solutions. By
contrast, when cis-OMCA molecules are generated by light irradiation and taken in the micelle-aggregate
solution, they slightly shorten distance between CTAB. Then, short wormlike micelles are formed and themicelle network development is restrained. As a result, the solution shows less viscoelasticity like a Newtonian
fluid. Based on the above mechanism, a viscoelasticity of CNO solutions can be changed by light irradiation
and also the concentration of cis- or trans-OMCA in the solution. The concentration ratio of trans-OMCA tocis-OMCA,  [%], which is an important index to evaluate viscoelasticity of CNO solution, is defined below:
ctrans(1) 100
ctrans  ccis
where ctrans and ccis are the mass concentrations [wt%] of trans-OMCA and cis-OMCA, respectively. By UVlight irradiation,  is decreased, and therefore the viscoelasticity is decreased. In this study, CNO solution of (trans-OMCA only) was prepared to be used in UV irradiation experiment. Besides, was
prepared by mixing half of trans-OMCA and cis-OMCA to be compared with the solution after UV irradiation.
trans-OMCA
cis-OMCA [L/(molcm)]

15000

10000

5000

0
240

280

320

360

wavelength [nm](a) OMCA photo-isomerization(b)  of trans-OMCA and cis-OMCA
Fig. 1 Schematics and molar absorption coefficient,  of OMCA .

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IFHT2016-1986
3. Apparatus
Figure 2 shows the schematic diagram and dimensions of the experimental setup comprised of a lightirradiation section and a heat transfer measurement section. The light irradiation section was installed upstreamof the measurement section. CNO solution flowing in the duct was irradiated and (partially) isomerized by UVlight before reaching to the measurement section where a heat transfer coefficient and a pressure loss were
measured to evaluate the effect of light irradiation in the upstream region.
At the light irradiation section, a 400W mercury lamp (SenLights HL400BH-8) covered by a water coolingjacket was inserted in a cylindrical chamber made by acrylic. CNO solution was fed to an inlet of the section
by a pressurized solution-stored tank, and irradiated by the lamp, flowing through the gap between the outerwall of cooling jacket and the inner wall of the chamber. To prevent heat of the lamp from reaching the CNO
solution, cooling water at 20 °C was circulated in the cooling jacket.
The measurement section had a square cross section of 5mm × 5mm, and comprised of an entrance duct, aserpentine channel and an exit duct. The serpentine channel consists of 10 semicircle units connectedperiodically in the streamwise direction. The inner and outer radii of curvature of the semicircle unit were Ri= 5mm and Ro = 10mm, respectively. The length of the serpentine channel along the streamwise direction, L,
was ca. 470mm. The entrance and the exit ducts were made of acrylic. The wall of the serpentine channel was
made of copper blocks, which was isothermally heated up by water circulation at constant temperature during
the experiment. CNO solution was supplied to the serpentine channel via the light irradiation section. The massof the fluid flowing from the outlet was weighed on an electronic scale in order to measure the mass flow rate,m [kg/s]. The mass flow rate was regulated by controlling the pressure in the tank. A K-type thermocouple
located at the center of the inlet was used to measure fluid temperature at the inlet, while that at the outlet wasmeasured by five K-type bare thermocouples evenly arranged in the height direction at the center of the channel.
Using these temperatures measured at the corresponding locations, the bulk mean temperatures of the fluid at
the inlet and outlet of the test section, Tb,i [°C] and Tb,o [°C], were estimated by weighted average method basedon the velocity profile of fully-developed flow in a square channel. The wall temperature, Tw [°C], was obtainedby averaging the temperatures measured by four thermocouples placed at the copper wall blocks. Pressure
sensors were installed at the entrance and the exit of the serpentine channel to measure a pressure loss, P [Pa].
The average heat transfer coefficient, hm [J/(m2･K･s)], and an average Nusselt number, Num, were defined asfollows:m cp
T  Tb, i(2)hm  ln wAs
Tw  Tb, o(3)
Nu m  hm Dh / kwhere As [m2] is the total area of heat transfer and Dh [m] is the hydraulic diameter of the channel. cp [J/(g･K)]
and k [J/(m･K･s)] indicate the specific heat capacity at constant pressure and thermal conductivity of the fluid,
respectively. We evaluate pressure loss penalty by Fanning’s friction factor, f defined as follows:
f 

Dh P
2
2 U m L(4)

where  [kg/m3] is the density of CNO solution. The flow rate is represented by modified Reynolds number,
Re*, which is defined as follows:
2 n
n
U Dh
Re  n-1 m(5)
8 K ( B  A / n) 2
where A and B are the geometrical constants, 0.2121 and 0.6766 at the rectangular flow, respectively [8]. The
values, n and K, referred to the constants used to express shear viscosity,  [Pa･s], as a function of shear rate, [1/s] ( = K  n-1). These two values were calculated from the results of static viscosity measurement using

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IFHT2016-1986
the rheometer. Physical properties (cp, k, ) of pure water are used for those of CNO solutions in the above
calculations with the consideration of temperature dependence.

Fig. 2 Apparatus.

4. EXPERIMENTAL CONDITIONSIn this research, viscoelasticity of CNO solution was evaluated by a relaxation time,  [s], determined using arheometer (ANTON PAAR, MCR301) equipped with a 50mm diameter cone plate. The measurement was
conducted while keeping the temperature of the measurement cell at 30°C.
At Re*~ O(1) the change in thermal properties by the light irradiation (i.e., isomerization) is considered to belarge [5], and thus it is desirable to perform experiments at such Re* condition. First, the CNO solution() was supplied to the channel with the mercury lamp off. The initial flow rate before the lightirradiation was set at Re* = 1.69 by adjusting a pressure in the tank. After the flow became stable, the lamp
was turned on. The light intensity of the lamp gradually increased over 300 seconds and after that maintained
a steady state. Also the flow rate increased with the light intensity during the warming-up time of lamp, which
resulted in a decrease in viscoelasticity and also pressure loss. This trend corresponds well with the decreaseof  implying the trans-to-cis transition, which was reported by the previous study [5, 6]. After the flow became
stable again, the measurement of heat transfer coefficient and pressure loss was performed for 100s, which isreferred to as Exp.1 hereinafter. After Exp.1, the pressure of the tank was manually increased to obtain a higher
flow rate. After the flow became stable, the measurement was performed for 100s again, which is referred toas Exp.2. The solution stably drained from the exit was collected in order to determine  of each solution
using the rheometer. To compare with the above results for the case of in-situ light irradiation, the measurementof heat transfer coefficient and pressure loss was performed for the trans- and cis- concentration adjustmentcase of CNO solution ( and ) without light irradiation. Note that the experiments for  and were conducted at various flow rate by changing the tank pressure. The flow rate, Q [mL/s], Re* of eachexperiment, and  of each solution are listed in Table 1.
Table 1 Flow conditions of heat transfer experiments and relaxation times of CNO solutions.
Experiment%
Exp. 1
Exp. 2

Q [mL/s]
0.47 ~ 2.07
0.41 ~ 2.28
1.45
2.08

4

Re*
0.66 ~ 11.00
0.36 ~ 9.83
4.30
7.80

 [s]
1.43
4.58
3.10
3.72

IFHT2016-1986
5. RESULTS
5.1 Viscoelasticity Figure 3 shows the relaxation time, , of CNO solutions plotted against the UV irradiation

time per unit volume, t* [s/mL]. The black squares plotted in Fig. 3 express  of CNO solutions at various t*
conditions after UV irradiation starts. The CNO solution given initially  was poured into an open
vessel located on a magnetic stirrer and irradiated by the same UV lamp which is used in the measurement
apparatus in Fig. 2. In this case, t* is calculated by t* = t / V, where t [s] is total light irradiation time and V [mL]is volume of a solution in the vessel. The open square indicates  as a reference. With the increase oft*,  of  decreased and converged to an almost constant value. This trend indicates that a photoisomerization from trans-OMCA to cis-OMCA progressed markedly in the early periods and the viscoelasticity
of solution correspondingly decreased with light irradiation. In this experiment, the photo-isomerization isconsidered to reach the equilibrium at t* = 30s/mL.
The red and green squares of Exp.1 and Exp.2 express  of CNO solutions collected at the channel exit. In thiscase, t* is calculated by t* = 1/Q. For both experimental conditions,  decreased compared to that of the initialsolution and the decrement became larger with increasing t*. This trend indicates that the photo-isomerization
was caused by in-situ light irradiation to the flow field using setup in Fig.1, and the viscoelasticity of solutions
decreased. Furthermore, this tendency accords well with the case of light irradiation to the solution in the open
vessel (the black square plots). This result implies that there is some possibility of predicting the  of the CNO
solutions undergoing in-situ light irradiation at the light irradiation section by using the results of black squares
plotted in Fig. 3.
10

 = 100 % (before UV)
 = 50 %
Exp.2

 [s]

Exp.1

1
0

10

20

30

40

t* [s /mL]

Fig. 3 Relaxation time,  of CNO solutions depending on the UV irradiation time per unit volume, t*.
5.2 Heat transfer and pressure drop Figures 4 (a) and (b) show, respectively, the average Nusselt number Num

and Fanning’s friction factor, f of CNO solution plotted against Re* in the case of , Exp.1 and Exp.2.
The solid line drawn in Fig. 4 (b) indicates the theoretically-obtained friction factor of steady flow in a square duct[8]. In Figs. 4 (a) and (b), Num and f of the case  (before UV) were totally larger than those of 
in all Re* region. This indicates that high viscoelasticity caused unsteady flow and enhanced heat transfer in thecase   100% while the low viscoelasticity caused steady flow and hardly enhanced heat transfer in the case.
In Exp.1, Num = 4.79 and f = 2.58 at Re* = 4.30. These values are much smaller than the case of  andalmost consistent with those of . This indicates that unsteady flow became steady with a decrease inviscoelasticity by light irradiation to the flow field, and as a result heat transfer and pressure loss decreased. In
Exp. 2, Num and f at Re* = 7.80 are also much smaller than those of the case , but, Num is still higher
than the case . This indicates that t* was decreased by increasing the flow rate compared to the Exp.1
and the decrement of  became smaller than the case of the Exp. 1, which will lead to the heat transfer control
by the degree of in-situ light irradiation. In both experiments, f are slightly lower than the case of ,although,  of Exp. 1 and Exp. 2 are higher than  (see Table. 2). These results still leave room for thefurther inspection about the relation of  to Num and f. Since there are few samples in this research, we will
perform the further experiments and confirm the reproducibility of the results.

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IFHT2016-1986

 = 100 % (before UV)

20

Exp.1

 = 50 %Theoretical value100

 = 100 % (before UV)

Exp.2

Exp.1

 = 50 %

Exp.2

f

Num

15

10

10

5

1
0

2

4

6

*

8

10

12

1

Re

(a)

Re*

10

(b)
Fig. 4 Nusselt number, Num and Fanning’s friction factor, f of CNO solution plotted against Re*.

6. CONCLUSIONS
Viscoelasticity: By in-situ light irradiation to the flow field, the relaxation time decreased compared to that of
the initial solution and the decrement became larger with increasing the light irradiation time.
Heat transfer and pressure drop: The decrease in viscoelasticity by light irradiation to the flow field possibly
modifies the flow structure, and as a result heat transfer and pressure loss decreased.

NOMENCLATURE
As
ccis
ctrans
Dh
f
hm
k
L
m

total area of heat t ransfer wall [m2]
weight density of cis-OMCA [wt%]weight density of trans-OMCA [wt%]hydraulic diameter of the channel [m]Fanning’s friction factor[-]
average heat transfer coefficient[J/(m2･K･s)]
thermal conductivity[J/(m･K･s)]
length along the serpentine channel [mm]mass flow rate
[ kg/s ]

Num
t*
Tb,i
Tb,o
Tw
P




average Nusselt number[-]
UV irradiation time per unit volume[s/mL]
bulk mean temperatures at the inlet [°C]bulk mean temperatures at the outlet [°C]wall temperature[°C]pressure loss[Pa]
cis-trans isomer existence ratio[%]a relaxation time[s]

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