Method and apparatus for augmentation of convection heat transfer in liquid

Electrodes are provided separated by spaces through which a liquid comes in and out, the electrodes being located 0.5 mm to 6.0 mm from the heat transfer surface in a liquid which has an electrical conductivity of 10.sup.-10 (1/(.OMEGA..multidot.m)) or more, the velocity of the flow being within the range of a Reynolds number for a laminar flow range, and a high-voltage direct current is applied to the electrodes to thereby produce turbulent components in the flow of the liquid to augment heat transfer between the liquid and the heat transfer surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and apparatus for the 
augmentation of convection heat transfer in a liquid which utilizes 
hydrodynamic forces produced by an electrical field, and more particularly 
to a method and apparatus for the augmentation of convection heat transfer 
in a liquid whereby, in a fluid transferring layer formed between a flow 
of liquid driven by an external source of pressure difference and a 
tubular member, such as in a heat exchanger tube, turbulence is produced 
only in the liquid in the fluid heat transferring layer formed in the 
vicinity of the tube's heat transfer surface, thereby suppressing the 
pressure loss of the flow while at the same time augmenting the heat 
transfer. 
2. Description of the Prior Art 
The degree to which convection heat transfer taking place between a heat 
exchange tube and a liquid flowing in the heat exchange tube can be 
augmented depends on how large the heat flux from the heat transfer 
surface to the liquid (or vice versa) can be made. 
Previously, convection heat transfer in the fluid heat transferring layer 
was augmented by creating turbulence in the thermal boundary layer by 
increasing the flow velocity of the liquid, increasing the Reynolds 
number, or by roughening the heat transfer surface and providing obstacles 
to the flow of the liquid. 
However, the conventional methods of augmenting convection heat transfer by 
producing turbulence in the flow of the liquid have had the following 
drawbacks. 
As the turbulence produced in accordance with the above methods of 
augmenting convection heat transfer increases the resistance to the flow 
of the liquid, there is an increase in the flow energy loss and the 
pressure loss which necessitates the use of a larger pump, for example, 
resulting in higher operating costs and increased energy consumption. When 
the pressure loss of the flow cannot be increased, the flow velocity has 
to be decreased. This produces a decrease in the heat transfer coefficient 
and, when the method is applied to a heat exchanger, a decrease in the 
heat exchange efficiency. 
In Japanese Patent Publication No. 59-66342 and U.S. Pat. No. 4,818,184, 
the present inventors disclose a method of utilizing hydrodynamic 
turbulence to agitate all of the fluid by providing surface electrodes and 
spatial electrodes arranged in opposition in the liquid and applying a 
high voltage across the electrodes to generate a jet stream in the liquid. 
Generating a high-velocity jet stream is an effective way of agitating all 
of the fluid but is difficult to apply to the augmentation of convection 
heat transfer of the liquid driven by the pressure difference through the 
production of turbulence only in liquid in the heat transferring layer in 
the vicinity of the heat transfer surface, such as when the pressure loss 
cannot be increased to an extent that will give rise to turbulence, or in 
the case of a slow flow in which a high degree of pressure loss is not 
possible. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method and apparatus for 
the augmentation of convection heat transfer in a liquid by producing 
turbulent components of the velocity only in the liquid of a thermal 
boundary layer while suppressing fluid pressure loss. 
For attaining the aforesaid object, the present invention provides 
electrodes separated by spaces through which a liquid comes in and out and 
spaced 0.5 mm to 6.0 mm from the heat transfer surface, producing a 
turbulence over the heat transfer surface of the liquid which has an 
electrical conductivity of 10.sup.-10 (1/.OMEGA..multidot.m)) or more at a 
velocity within a Reynolds number of laminar flow range, and applying a DC 
voltage to the electrodes to produce turbulent components in the liquid 
flowing in the thermal boundary layer to thereby augment convection heat 
transfer between the liquid and the heat transfer surface. 
In the arrangement of the invention as described above, as turbulence is 
produced only in the liquid flowing in the thermal boundary layer, an 
efficient transfer of heat from the thermal boundary layer to the liquid 
can be achieved with virtually no loss of fluid pressure in the viscous 
boundary layer. 
These and other objects and features of the invention will be better 
understood from the following detailed description made with reference to 
the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the basic structure of the apparatus for the augmentation of 
convection heat transfer in a liquid in accordance with the present 
invention. With reference to FIG. 1, electrodes 2 spaced apart by a 
prescribed distance are disposed opposite a heat transfer surface 4 of a 
heat transfer member 1 in which a liquid 3 flows. As well as transferring 
heat to the liquid, the heat transfer surface 4 of the heat transfer 
member 1 also functions as a ground electrode, and therefore it is 
constituted of a material which has good electrical and thermal 
conductivity. 
Preferably the electrodes 2 disposed opposite the heat transfer surface 4 
are configured in a way that does not produce increased flow resistance. 
It is also necessary to separate the electrodes by spaces 2' to allow an 
exchange of momentum and heat to take place in the liquid 3 on both sides 
of the electrodes 2. There is no particular limitation on the shape of the 
electrodes, other than that the configuration should not be one that gives 
rise to the formation of a jet stream in the liquid. Thus, the electrodes 
may be configured as a multiplicity of metal wires stretched in parallel, 
as a metal mesh, or as perforated metal plates. In view of the 
requirements described above, electrodes of metal mesh are particularly 
suitable, or electrodes of metal wire, which would enable the 
cross-sectional area to be reduced, decreasing resistance to the liquid, 
and the spaces 2' to be increased. To prevent jet streams arising in the 
liquid, wire electrodes have to be spaced a uniform distance apart, while 
in the case of perforated plate electrodes the shape and the dimensions of 
the spaces 2' have to be substantially identical. 
Preferably the space between the heat transfer surface 4 and the electrodes 
2 is about the same as, or slightly larger than, the thickness of a 
thermal boundary layer 6 formed in the vicinity of the heat transfer 
surface 4 in contact with the liquid 3 via which the transfer of heat 
takes place, and about the same as, or slightly thinner than, the 
thickness of the viscous boundary layer. That is, as shown in FIG. 2, in 
the vicinity of the heat transfer surface 4 there are a thermal boundary 
layer 6 that is the extent of the range of thermal conductivity and a 
viscous boundary layer 7 that is the extent of the range of the viscosity 
of the liquid. 
The thickness .delta. of the viscous boundary layer 7 is given by 
.sqroot.(.nu..multidot.X)/U, where .nu. is the kinematic viscosity of the 
liquid, X is the length of the heat transfer member and U is the flow 
velocity of the liquid driven by an external source of pressure difference 
so that with a Reynolds number of Re=(U.multidot.X)/.nu., the thickness 
.delta. of the viscous boundary layer will be X/(.sqroot.Re). 
The ratio of the thickness of the thermal boundary layer to that of the 
viscous boundary layer (viscous boundary layer thickness/thermal boundary 
layer thickness) is shown by the Prandtl number 
(=.nu./(.lambda./.rho.C.rho.)), where .lambda. is thermal conductivity, 
.rho. is density and C.rho. is specific heat at constant pressure. The 
Prandtl number of a Freon (CFC or HCFC) is around 4 and that of oil is 
around 100; the Prandtl number of the subject fluid, in which the thermal 
boundary layer is thinner than the viscous boundary layer, is normally no 
more than a fraction of 1. 
The thickness of the thermal boundary layer 6 in normal convective heat 
transfer is within the range of the laminar flow that has been influenced 
mainly by the viscosity over the total flow (a Reynolds number of up to 
several thousand, when the heat transfer surface is a flat plate), or 
around 0.1 mm to 3.0 mm, and hence the gap between the electrodes 2 and 
the heat transfer surface 4 preferably is around 0.5 mm to 6.0 mm. 
The characteristic charge relaxation time tc of the liquid (heat 
transferring medium) which receives the heat transferred from the heat 
transfer surface 4 is represented as a ratio of the electrical 
conductivity .sigma.e and the dielectric constant .epsilon., thus 
(.epsilon..multidot..epsilon.o)/.sigma.e . In the equation, .epsilon.o is 
the dielectric constant in a vacuum. It is preferable that the charge 
relaxation time is smaller than the characteristic flow time D/U (D being 
heat transfer surface and U the flow velocity). For example, if a tube the 
inside diameter of which is 10 mm is taken as the length of the heat 
transfer surface and 100 mm/sec is the mean flow velocity, the 
characteristic flow time D/U would be 100 ms, so when the dielectric 
constant .epsilon. is 2, if the electrical conductivity .sigma.e is larger 
than 2.times.10.sup.-10 (1/(.OMEGA..multidot.m)), the charge relaxation 
time of the liquid would be smaller than 100 ms, where the effects of 
applying electric fields become marked. Liquids having such properties 
include R123, a Freon substitute, silicon oil, and transformer oil. 
Preferably the flow velocity of the liquid over the heat transfer surface 4 
is within the range of a laminar flow with a low pressure loss. For 
example, when the heat transfer surface is a round duct, with the Reynolds 
number (Re=(U.multidot.X)/.nu.) being a function that is proportional to 
the flow velocity, the target is a Reynolds number in the range 2000 to 
4000, and when the heat transfer surface is a flat plate, the target is a 
Reynolds number in the range below 5.times.10.sup.5. 
If the flow velocity of the liquid is higher than this range there will be 
a transition to a turbulent flow and an increase in the pressure loss. If 
the flow velocity is smaller than this range, the heat transfer 
augmentation effect will be increased by just the amount concerned. 
With the above configuration, if a voltage of 1000 to 3000 volts is applied 
between the heat transfer member 1 and the electrodes 2, as shown in FIG. 
2 (which shows when the negative is applied to the heat transfer member 1 
and the positive to the electrodes 2), ions from the electrodes and ions 
present in the liquid will move in the space between the electrodes 2 and 
the heat transfer surface 4, and the Coulomb force exerted on the ions by 
the electrical field produces turbulent components of velocity in the 
liquid in the thermal boundary layer 6, giving rise to a turbulent flow. 
As a result, heat transfer is augmented as near-turbulent heat transfer 
and, although the flow resistance increases somewhat as a result of a 
decrease in the thickness of the viscous boundary layer 7, owing to the 
slow velocity of the main flow, in the region of the main flow there is an 
attenuation of the turbulent components, i.e., of the time fluctuation 
components of the flow velocity, so that there is little overall increase 
in the pressure loss, which remains small compared to turbulent heat 
transfer realized by the usual method. 
Since virtually no movement of ions would be present except between the 
heat transfer surface and the electrodes, there occurs virtually no 
velocity fluctuation, i.e. no turbulence. Furthermore, ion movement 
between the heat transfer surface 4 and the electrodes 2 is perpendicular 
to the mean flow, so there is an increase in heat and momentum exchange 
perpendicular to the flow. 
With reference to FIG. 3, at a point X.sub.1 in the temperature 
distribution in the flow of the liquid 3, driven by an external source of 
pressure difference in the region of the heat transfer surface 4 the 
temperature To of the liquid changes to the temperature Tw of the heat 
transfer surface 4 (the length of the arrows indicates the magnitude of 
the temperature), and there is almost no change in the upper part of the 
thermal boundary layer 6. At a point X.sub.2 in the velocity distribution 
of the flow of the liquid 3, velocity at the heat transfer surface is zero 
and there is a small velocity near the heat transfer surface, the velocity 
gradually increasing towards the outer edge of the viscous boundary layer. 
Thus, in accordance with this invention, turbulence is produced 
hydrodynamically only in the liquid in the thermal boundary layer, 
enabling the thickness of the thermal boundary layer to be decreased, 
providing a low-pressure-loss, high-efficiency-heat-transfer convection 
heat exchange apparatus in which a lower main flow velocity can be used to 
obtain the same heat transfer coefficient. 
FIG. 4 shows the temperature distribution in a tube 8 in accordance with 
the present invention, in which only the liquid in the vicinity of the 
inner wall of the tube transfers heat from the inner wall and undergoes a 
sharp change. For reference, the temperature distribution at the point the 
application of the voltage is stopped is shown by the dashed line. As 
shown in FIG. 4 (b), which illustrates the velocity distribution of the 
liquid in the tube, a relatively sharp velocity gradient exists only near 
the inner wall of the tube, the velocity increase being gradual going 
towards the center. The dashed line shows the velocity distribution in the 
liquid when no voltage is being applied. 
Since the viscous effects are relatively large in the boundary layer, in 
which there is a change of temperature, the flow velocity is largely 
reduced. As a consequence of the small transportation rate of the viscous 
boundary layer, the degree of convection heat transfer from the wall is 
determined by the state of the liquid flow. Thus, an effective way is to 
promote transport from the wall by utilizing the turbulent components of 
the flow to increase the transport phenomena derived from the creation of 
a turbulent flow in the thermal boundary layer. 
As one example, heat transfer experiments were conducted in which two 
copper heat transfer surfaces 20 mm apart were heated to a heat 
differential of 5 K relative to the liquid, a multiplicity of wires 0.3 mm 
in diameter and each separated from the next by a distance of 10 mm were 
positioned 6 mm away from the lower of the heat transfer surface, a flow 
of a liquid consisting of Furonsorubu AE (R 113: 96 wt%; ethanol: 4 wt%) 
was produced across the heat transfer surface, and a direct current was 
applied to the electrodes, using a Reynolds number of 1000. The results 
are shown in FIG. 5. The heat transfer coefficient was about 24.9 
W/(m.sup.2 .multidot.K) when no electricity was applied. With a direct 
current of 3 kV, the heat transfer coefficient rose to around 320 
W/(m.sup.2 .multidot.K), and to about 650 W/(m.sup.2 .multidot.K) with a 
direct current of 4 kV, or over a 25-fold increase in the coefficient 
compared to when no electricity is applied. 
From the foregoing description, the present invention utilizes hydrodynamic 
forces to produce the turbulent components and thereby induces turbulence 
only in liquid within the thermal boundary layer, thereby suppressing the 
pressure loss and augmenting the heat transfer process by a change to 
turbulent heat transfer. 
Applying the invention to heat exchangers enables the mean flow velocity of 
the liquid to be reduced. This means that the pressure loss can be made 
lower, so a less powerful pump can be used. This makes it particularly 
suited to pressure loss suppression applications, and as the area of the 
heat transfer surface can be reduced, the heat exchanger can be made more 
compact.