BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present disclosure relates to heat exchangers, and more particularly to heat
transfer tubes used with heat exchangers.
2. Description of Related Art
[0002] In-tube boiling is used in nearly all two-phase power and thermal management systems,
such as, Rankine power cycles, HVAC vapor cycles thermal management, two-phase thermal
buses and in many chemical processing applications. Boiling heat transfer is used
to remove heat in cooling applications, for example, electronics cooling. High heat
flux applications like high power lasers and microwaves rely on thermal management
systems that employ two-phase cooling.
[0003] In-tube boiling can have very high heat transfer coefficients resulting in low wall
temperature to fluid saturation temperature differences. However, in-tube boiling
has several potential limitations. First, at high heat fluxes the voluminous vapor
generated at the heated wall can block the free stream liquid from rewetting the wall.
This well studied phenomenon is called Critical Heat Flux (CHF), or burnout. Secondly,
boiling can be orientation and gravity sensitive. Beyond shear forces, gravity is
the key force that motivates bubbles to leave the hot boiling surface. In adverse
orientations, buoyancy forces tend to keep the vapor on the wall and thereby reduce
the heat transfer coefficient and accelerate CHF conditions. For example, in reduced
gravity environments buoyant forces do not exist and boiling heat transfer is severely
limited. Many gravity insensitive geometries, like swirl flow inserts and curved channels
that use centrifugal forces as a substitute for gravity have been used with limited
success.
[0004] Such conventional methods and systems have generally been considered satisfactory
for their intended purpose. However, there is still a need in the art for an improved
heat transfer tubes. The present disclosure provides a solution for this need.
SUMMARY OF THE INVENTION
[0005] A heat transfer tube includes a tube wall defining a central axis in a lengthwise
direction of the tube. The tube wall includes a fluid inlet and fluid outlet for directing
a coolant into and out of the tube. A hollow cone is positioned within the tube wall
aligned with the central axis having an interior and exterior. A plurality of orifices
are defined through the cone configured to provide a separation of liquid coolant
inside the cone from vapor outside the cone within the tube wall.
[0006] The orifices can be configured to form impingement jets of liquid directed from the
interior of the cone towards the tube wall. The orifices can further be configured
to allow vapor flow circumferentially and/or axially between the jets thereby allowing
liquid coolant to impinge on the tube wall. The orifices can be dispersed throughout
a length of the central cone.
[0007] The central cone can converge down in cross-sectional area in a downstream direction
from an inlet end of the cone. The downstream end of the cone can be closed such that
all fluid flow from the interior of the cone passes through the orifices and exits
the tube wall from the outlet of the tube wall. Flow area of liquid coolant can decrease
as a flow area of two-phase flow increases thereby making volumetric flow uniform
within the tube. An outer surface of the tube is configured to be in thermal communication
with a heat source. A heat exchanger can include a plurality of heat transfer tubes
as described above.
[0008] A heat transfer device including a housing defining a central axis. The housing includes
a fluid inlet and fluid outlet for directing a coolant into and out of the housing.
A hollow insert within the housing aligned with the central axis having an interior
and exterior. A plurality of orifices defined through the insert configured to provide
a designed distribution of liquid coolant from the insert to the annular space that
provides coolant for boiling at the outer surface and carries a vapor or vapor-liquid
mixture axially outside the insert within the housing.
[0009] These and other features of the systems and methods of the subject disclosure will
become more readily apparent to those skilled in the art from the following detailed
description of the preferred embodiments taken in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] So that those skilled in the art to which the subject disclosure appertains will
readily understand how to make and use the devices and methods of the subject disclosure
without undue experimentation, preferred embodiments thereof will be described in
detail herein below with reference to certain figures, wherein:
Fig. 1 is a cross-sectional view of a conventional boiling tube; and
Fig. 2 is a cross-sectional view of an exemplary embodiment of a heat transfer tube
constructed in accordance with the present disclosure, showing a hollow cone within
the tube wall.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0011] Reference will now be made to the drawings wherein like reference numerals identify
similar structural features or aspects of the subject disclosure. For purposes of
explanation and illustration, and not limitation, a partial view of an exemplary embodiment
of a heat transfer tube in accordance with the disclosure is shown in Fig. 2 and is
designated generally by reference character 100.
[0012] Fig. 1 illustrates a conventional boiling tube 10. In a horizontal orientation, several
flow patterns develop as the quality (vapor fraction of total flow) increases. At
low qualities bubbly flow dominates, followed by slug and plug (intermittent liquid
and vapor bridges), annular flow and then a mist. These changes occur in a direction
from left to right as oriented in Fig. 1
[0013] With reference to Fig. 2 a heat transfer tube 100 is shown in accordance with the
present disclosure. The tube 100 can be part of a heat exchanger that is in communication
with a heat source 120, for example, an electronic heat, a high heat flux load like
a laser, or a hot fluid to be cooled. The tube 100 improves the boiling process while
reducing heat transfer area, system weight, and increases system capabilities. The
tube 100 includes a tube wall 102 defining a central axis A-A in a lengthwise direction.
The tube wall 102 includes a fluid inlet 104 and fluid outlet 106 for directing a
coolant into and out of the tube 100. A hollow cone 110 is positioned within the tube
wall 102 aligned with the central axis. As shown in Fig. 2, the cone 110 converges
down in a cross-sectional area in a downstream direction from the inlet end 104a of
the cone 110 toward outlet 106.
[0014] A plurality of orifices 112 are defined through the cone 110 each extending from
an interior 114 of the cone 110 to an exterior 116 of the cone 110. The downstream
end 106a of the cone 110 is closed such that all fluid flow from the interior 114
of cone passes through the orifices and exits the tube 100 through the outlet 106.
The plurality of orifices 112 are configured to provide a separation of liquid coolant
within the cone 110 from vapor outside the cone 110 within the tube wall 102. More
specifically, the orifices 112 are configured to form impingement jets of fluid directed
from the interior 114 of the cone 110 towards the tube wall 102. The impingement of
the jets increases the heat transfer coefficient, compared to conventional systems
as shown in Fig. 1, and more importantly, keep the wall 102 wetted thereby greatly
increasing critical heat flux (CHF). Vapor flows circumferentially and/or axially
between the jets allowing the liquid coolant to impinge on tube wall 102. Very little
distortion of the jets and degradation of its velocity occurs in areas of high quality
(downstream) and void fraction (volume fraction of vapor). These "drier" areas are
those that benefit most with respect to heat transfer coefficient and suppressing
CHF, when compared to conventional systems as shown in Fig. 1. In regions where the
two-phase flow is of lower quality (upstream) the jets may have less of an impact
on heat transfer but are not at risk of CHF.
[0015] If the heat flux distribution is not circumferentially or axially symmetric, two
design possibilities can be employed. First, the orifice distribution can also be
made asymmetric so that orifice pattern produces a mass distribution for a better
match the local heat fluxes. Secondly, the liquid distribution cone can be situated
non-concentrically, with the orifice distribution providing a greater mass flux in
the region that is closest to the heated outer wall. This embodiment provides better
cooling where locally needed and extra flow space in other arcs to carry the spent
vapor or liquid-vapor mixture.
[0016] The plurality of orifices 112 are disposed throughout the length of the central cone
110. The flow area of the orifices determines the jet velocity and pressure required
for a given mass flow on an application by application basis. Fewer or smaller orifices
will have higher velocity jets but a greater pressure drop. The size, number and distribution
of the orifices can be optimized for any given application.
[0017] The impingement jets also make the heat transfer insensitive to orientation and gravity
level. This feature and makes the device ideal for applications like micro-gravity.
Additionally, due to the converging cross-sectional area of the cone 110, the flow
area of the liquid coolant is decreasing while two-phase flow area is increasing.
Pressure drops are reduced and the heat transfer is optimized by making the volumetric
flow velocity more uniform in both regions comparted to conventional systems. Furthermore,
the temperature of the liquid coolant supplied to the tube 100 does not significantly
vary along the tube 100. To have this "fresh" coolant everywhere is advantageous,
especially if the inlet flow is subcooled (below the saturation temperature). Subcooling
increases the heat transfer coefficient and CHF.
[0018] This concept of impingement boiling may also be extended to a planar geometry. Fig.
2 can illustrate the planar concept as well as the cylindrical. For example, if the
cross-section represents a slice in a two dimensional plane, then the heat transfer
walls are flat surfaces rather than a cylindrical wall of a tube. Planar embodiments
are suitable for many two-phase applications in thermal management, power and process
heat transfer. They are also a good approach for phase management in reduced gravity
environments.
[0019] The methods and systems of the present disclosure, as described above and shown in
the drawings, provide for a device for increasing heat transfer with superior properties
including the use of impingement gets to decrease central heat flux. While the apparatus
and methods of the subject disclosure have been shown and described with reference
to preferred embodiments, those skilled in the art will readily appreciate that changes
and/or modifications may be made thereto without departing from the scope of the subject
disclosure.
1. A heat transfer tube (100), comprising:
a tube wall (102) defining a central axis in a lengthwise direction of the tube (100),
the tube wall (102) including a fluid inlet (104) and fluid outlet (106) for directing
a coolant into and out of the tube (100);
a hollow cone (110) within the tube wall (102) aligned with the central axis having
an interior and exterior; and
a plurality of orifices (112) defined through the cone (110) configured to provide
a separation of liquid coolant within the cone (110) from vapor outside the cone (110)
within the tube wall (102).
2. The tube of claim 1, wherein the orifices (112) are configured to form impingement
jets of fluid directed from the interior of the cone (110) towards the tube wall (102).
3. The tube of claim 2, wherein the orifices (112) are configured to flow vapor circumferentially
and axially between the jets thereby allowing liquid coolant to impinge on the tube
wall (102).
4. The tube of any preceding claim, wherein the central cone converges down in cross-sectional
area in a downstream direction from an inlet end of the cone (110).
5. The tube of claim 4, wherein the downstream end of the cone (110) is closed such that
all fluid flow from the interior of the cone (110) passes through the orifices (112)
and exits the tube wall (102) from the outlet (106).
6. The tube of any preceding claim, wherein the orifices (112) are dispersed throughout
a length of the central cone.
7. The tube of any preceding claim, wherein a flow area of liquid coolant decreases as
a flow area of two-phase flow increases thereby making volumetric flow uniform within
the tube wall (102).
8. The tube of any preceding claim, wherein an outer wall of the tube (100) is configured
to be in communication with a heat source.
9. A heat exchanger, comprising:
a plurality of heat transfer tubes (100), each heat transfer tube (100), comprising:
a tube wall (102) defining a central axis in a lengthwise direction of the tube (100),
the tube wall (102) including a fluid inlet (104) and fluid outlet (106) for directing
a coolant into and out of the tube (100);
a hollow cone (110) within the tube wall aligned with the central axis having an interior
and exterior; and
a plurality of orifices (112) defined through the cone (110) configured to provide
separation of liquid coolant and vapor within the tube (100).
10. A heat transfer device, comprising:
a housing defining a central axis, the housing including a fluid inlet (104) and fluid
outlet (106) for directing a coolant into and out of the housing;
a hollow insert within the housing aligned with the central axis having an interior
and exterior; and
a plurality of orifices (112) defined through the insert configured to provide a designed
distribution of liquid coolant from the insert to an annular space that provides coolant
for boiling at an outer surface of the insert and carries a vapor or vapor-liquid
mixture axially outside the insert within the housing.