Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
1Development ofCompact and High-performance Heat Exchangers
Using Metal Additive Manufacturing
HIROYUKI NAKAHARAI*1

KOICHI TANIMOTO*2

YOICHI UEFUJI*1

YUTA TAKAHASHI*3

MASAYA HATANAKA*4TOSHIFUMI KANNO*5

Improvements in metal additive manufacturing (AM) technology in recent years haveenabled forming very precise shapes. By utilizing this technology, it is possible to realize
unprecedented high-performance heat exchangers. Heat exchangers are used in a variety ofproducts, and for example, in products for mobility such as vehicles, space equipment and ships,reducing the size and weight of onboard equipment they have can contribute to improving fuelefficiency and enhancing dynamic performance. We have examined prototypes, performance tests,
and quality assurance of heat exchangers for mobility products utilizing AM technology(hereinafter "AM heat exchanger(s)"), which this report introduces, and have confirmed that theweight and volume can be reduced by half compared to the conventionally used heat exchangers.

|1. Introduction
Improvements in AM technology in recent years have enabled fabrication of very precise andcomplex structures that could not be made with conventional machining technology. By applying
this technology to heat exchangers, it is possible to arrange precise heat transfer promoters in theinternal channels and to design the channels themselves with complex curved surfaces, thereby
increasing the amount of heat exchanged per pressure drop compared to conventional heatexchangers manufactured by machining. Furthermore, the use of AM technology improves thedegree of freedom in the shape of the heat exchanger itself, enabling compact packaging combinedwith other equipment by designing the shape of the heat exchanger according to the installation
space. Thus, the ability to not only reduce the size and weight of a heat exchanger alone, but also
minimize the packaging space including other equipment, is one of the strengths of Mitsubishi
Heavy Industries, Ltd. (MHI), which designs and manufactures heat exchangers as well as entire
systems.
In addition, quality assurance is also essential for using AM heat exchangers with suchexcellent features in actual products. The AM heat exchanger described in this report is built using
the powder bed method, which enables precise build. In general, various factors such as laser
power, scan speed, and powder properties affect the build quality. Unless these factors are properly
controlled, defects and incomplete fusion will occur within the built metal, which will result in aloss of strength.
MHI has been working on the development of AM heat exchangers focusing on performance
improvement and quality assurance. This report introduces examples of the development and future
prospects.

*1

Research Manager, Heat Transfer Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Fellow Staff, Heat Transfer Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*3Heat Transfer Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*4

Research Manager, Manufacturing Technology Research Department, Research & Innovation Center, Mitsubishi Heavy Industries,*5

Strength Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
2|2. Structure and features of AM heat exchanger
As mentioned above, MHI is developing AM heat exchangers of various concepts. Thischapter introduces some of them.

2.1

Mini channel counterflow AM heat exchanger

Figure 1 shows a counterflow AM heat exchanger configured with mini channel. This heatexchanger has mini channel to increase the heat transfer area per unit volume, and contains a
precise heat transfer prompter inside the channels to increase the amount of heat exchanged per thesame pump power compared to smooth channel structure. The header is designed so that the flow is
distributed evenly to each channel and that the performance degradation due to flow deviation isabout 1% under the main operating environment.

Figure 1

Mini channel counterflow AM heat exchanger

Figure 2 shows a test piece for elemental testing to evaluate the channel performance. The
number of channels was set to five on both the high and low temperature sides, and the heattransfer coefficient and pressure drop characteristics of the channels were obtained using thesmallest element. Tests were conducted using water, oil, and air as media to evaluate the heattransfer and pressure drop characteristics, as shown in Figure 3, and design equations weredeveloped (1). Here, the Reynolds number is a dimensionless number that expresses the ratio ofviscosity and inertia, but in the same fluid, it expresses how much the flow rate is. The Nusselt
number is a dimensionless number that represents the magnitude of heat transport (heat transfercoefficient) between the fluid and the bulkhead and varies with the Reynolds number. Since theNusselt number also varies with the viscosity, specific heat, and thermal conductivity of the fluid, adimensionless number called the Prandtl number, which is the ratio of the kinematic viscosity andtemperature diffusivity, is generally used in addition to the Reynolds number when formulatingheat transfer characteristics. The Prandtl numbers of the fluids used in this test, water, oil, and air
are widely different from each other, and the design equations for a wide range of Prandtl numbers
were developed in this study.

Figure 2 Test piece for elemental testingMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
3

Figure 3

Heat transfer and pressure drop characteristics obtained from test

We are able to design heat exchangers with appropriate specifications by using AM heatexchanger performance evaluation equations developed based on elemental tests as above.
Figure 4 compares the designed and manufactured AM heat exchanger for mobility with aconventional heat exchanger; the AM heat exchanger is half the weight and volume of theconventional plate type heat exchanger.

Figure 4 Comparison between conventional heatexchanger and AM heat exchanger

2.2

Catalyst-integrated AM heat exchanger

As mentioned in the introduction of this report, utilizing AM technology for heat exchangers
improves the degree of freedom of the shape of the heat exchanger itself, allowing the design of aheat exchanger shape that is appropriate for the installation space. This enables compact packaging
combined with other equipment. As an example, a catalyst-integrated heat exchanger is introduced
here.
MHI designs and manufactures various products using catalysts, many of which use heatexchangers to control the temperature in the range suitable for the catalytic reaction. Therefore,
piping for connecting the heat exchanger to the catalyst is necessary, and naturally, space is needed
to route the piping. As a solution for this, we have developed a catalyst-integrated AM heatexchanger that greatly reduces the package space, including piping. Figure 5 shows a photographof its structure and appearance. In this example, the incoming gas is heated by the heat exchanger
and electric heater and fed to the catalyst. Then the gas is cooled by the low-temperature gas thatflows into the system to bring it close to the temperature of the inflow gas, and discharged.
We have completed prototyping and performance testing of this AM heat exchanger andhave confirmed that it performs as expected. Figure 6 compares the preliminary design values ofthis catalyst-integrated AM heat exchanger with the performance test results. The medium used
was air for both the high and low temperature sides, and varied between 20 and 25°C for the lowtemperature side and between 170 and 230°C for the high temperature side. The design values werein good agreement with the measured values from the test. The difference in heat exchange andpressure drop was about ±5% and ±10%, respectively. Based on the results of this performance
test, the designed catalyst-integrated AM heat exchanger is expected to reduce the conventional
package space (including piping routing) by half.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
4

Figure 5

Catalyst-integrated AM heat exchanger

Figure 6

Comparison of design values and test results

2.3

In-pipe embedded AM heat exchanger

Figure 7 shows an AM heat exchanger designed to be embedded inside a pipe. Recoveringand reusing waste heat is very important from the perspective of improving the performance andefficiency of products, but some products having many components packaged compactly may nothave enough space to add a heat exchanger. In such cases, heat exchangers that can be installedinside pipes are considered to be very useful.

Figure 7

In-pipe embedded AM heat exchanger

Figures 8(a) and 8(b) show the photographs of prototypes of in-pipe embedded AM heatexchangers. Figure 8(a) shows the core of the in-pipe embedded AM heat exchanger, whichcorresponds to the part labeled "Heat exchanger core" in Figure 7. The channels inside the core areMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
5

arranged so that the high-temperature fluid and low-temperature fluid flow in counter directions,with the first fluid passing straight through the core inside the piping and the second fluid flowing
through a slit on the side into the core. In the case of the prototype shown in Figure 8(b), the first
fluid flows inside the hexagon and the second fluid flows in the gap around it. Generally, when thevolume flow rates of the first and second fluids differ greatly, it is necessary to increase thecross-sectional area of the channel with the larger volume flow rate. This heat exchanger, which
has a large difference in the channel cross-sectional area between the inside of the honeycomb andthe gap, is suitable for fluids with significantly different volumetric flow rates (e.g., liquid and gas).
Currently, we are also developing a high-performance honeycomb heat exchanger in which a heattransfer enhancing structure created by topology optimization is placed in the gap. Analytical
calculations of this heat exchanger have shown that it is half the volume of a typical
double-tube-type heat exchanger.

Figure 8 Prototypes of in-pipe embedded AM
heat exchanger|3. Quality assurance initiatives
For mounting AM heat exchangers in products, it is necessary to guarantee quality according
to the product. For example, in the powder bed method currently used for building AM heatexchangers, the build quality varies depending on various conditions, such as laser output, scanspeed, powder particle size, control method, and build posture. Figure 9 compares built products inthe case where the heat input is intentionally excessive by actually varying the laser power and in
the case where the heat input is appropriate. When the heat input is excessive, the amount of metal
vapor generated during powder melting is entrained, resulting in numerous voids occurred asshown in the figure. In contrast, it is known that insufficient laser power also results in voids due topoor fusion. If such voids exist, depending on the environment in which the product is used, cracks
may propagate starting from the voids due to thermal stress, vibration, or other factors. Therefore, it
is important to accurately understand the impact of each factor on quality and to properly control
the build conditions. We conduct the build under various conditions in advance to understand theeffects of these factors, and have accumulated the necessary data for design through strength
evaluation tests of the build material, etc.

Figure 9

Cross-section of AM productFigure 10 shows a contour diagram of Mises equivalent stress, which is the result of thecalculation of thermal stresses caused by the temperature distribution and thermal deformation

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
6

constraints at the four corner fixation points in the heat exchanger shown in Figure 1 above by
FEM analysis through modeling up to the internal structure. The photograph in Figure 10 shows theappearance of a fatigue element test piece that simulates the channel of the AM heat exchanger andwas manufactured under the same build conditions as the heat exchanger. The test piece in thephotograph is shown after being ruptured in a test. By conducting such tests, we have accumulated
basic data for evaluating the strength of AM heat exchangers.
Thus, we ensure reliability in designing AM heat exchangers by conducting life evaluations
based on detailed FEM analysis and test data after understanding product-specific requirements
such as vibration conditions, the number of start-ups and shutdowns, and thermal stresses.

Figure 10 Strength reliability evaluation by elemental test andFEM analysis|4. Conclusion
This report introduced our initiatives to improve the performance and reduce the size of heatexchangers by utilizing AM technology, which has been rapidly developing in recent years. It also
introduced examples of integrating heat exchangers into other equipment to reduce its packaging
space for equipment including the heat exchanger.
MHI has been conducting a series of studies from concept creation, prototyping, performance
evaluation, vibration/strength evaluation, and quality assurance, and have already achieved a 50%
reduction in weight and volume for some products compared to conventional heat exchangers. In
addition, we are continuing to work on further performance improvement and size reduction byutilizing topology optimization methods and other techniques. We will continue to contribute to theadvancement of energy management (improvement of efficiency, reduction of energy consumption,etc.) for our variety of products through high-performance, compact, and lightweight heatexchangers.

References(1)Shunsaku Eguchi et al., Heat Transfer Characteristics of the Flow in Ribbed Channel，The 58th National
Heat Transfer Symposium of Japan Proceedings (2021-5)