EP3385656A1

EP3385656A1 - Heat exchanger element and method for manufacturing same

Info

Publication number: EP3385656A1
Application number: EP17401041.3A
Authority: EP
Inventors: Steffen Grohmann; David GOMSE; Bertrand Dutoit
Original assignee: Karlsruher Institut fuer Technologie KIT
Current assignee: Karlsruher Institut fuer Technologie KIT
Priority date: 2017-04-07
Filing date: 2017-04-07
Publication date: 2018-10-10
Anticipated expiration: 2037-04-07
Also published as: EP3385656B1; WO2018184712A1; US20210018281A1

Abstract

The present invention provides a heat exchanger element (1) and a method for manufacturing a heat exchanger element (1) being in contact with a gas (3), wherein said heat exchanger element (1) comprises at least one solid surface (2). Said solid surface (2) is coated with at least one layer (4) of a predetermined material, said layer being suitable to enhance the heat transfer between the solid surface (2) and said gas (3) by thermoacoustic impedance matching.

Description

The invention refers to a heat exchanger element and a method for manufacturing said heat exchanger element.
Heat transfer enhancement between solid surfaces and fluids is relevant for heat exchangers, which are applied in all kind of technical applications and their performance has a large impact on the overall efficiency. Heat exchangers transfer thermal energy (heat) from a solid heat source to a cooling fluid, from a warm fluid to a solid heat sink, or between several fluids that are separated by a solid heat exchanger wall. Classical heat transfer enhancement techniques incorporate one or a combination of either increase of heat exchanger surface, increase of fluid velocities and promotion of turbulence in the fluid-side thermal boundary layer or influencing surface wettability and nucleation site activity in case of boiling and condensation (R. L. Webb, "Principals of enhanced heat transfer", Wiley, 1994).
For an increase of heat exchanger surface, surface extensions with fins are used. Most designs also promote turbulence in the boundary layer by segmenting the enhanced surfaces.
Another enhancement technique is to increase fluid velocities: Insert devices provide a periodic acceleration/deceleration of the fluid as well as turbulence in the fluid-side thermal boundary layer by periodically changing the flow cross-section. Swirl flow devices, for example, increase the fluid velocity by forcing the fluid on a swirling or helical streamline. In addition, some active techniques such as acoustic or electric fields and surface vibration are used to promote turbulence in the fluid. The artificial increase of the surface roughness mainly effects the formation of a thermal boundary layer, but has negligible influence on the surface area. The increase of roughness may be achieved by surface coatings incorporating relatively large particles.
Influencing surface wettability and nucleation site activity in case of boiling and condensation is yet another enhancement possibility. This can be achieved by surface coatings that change the surface energy of the heat exchanger wall. The surface energy influences the wetting angle, which has a significant influence on two-phase heat transfer in both boiling and condensation. Porous surfaces are also used to enhance nucleate boiling by providing artificial nucleation sites.
In Connor et al. "A dielectric surface coating technique to enhance boiling heat transfer from high power microelectronics", IEEE Transactions on Components, Part A:, 18(3), 1995, an example for enhancement of boiling heat transfer in power micro-electronic devices by surface coatings with solid particles is disclosed: Al₂O₃ particles and diamond particles of 1 to 12 µm size are bonded to the surface with paint in layers up to 50 µm thickness. The microporous surface provides additional nucleation sites and enhances the nucleate boiling heat transfer by approximately factor two. Also, US 2007/0230128 A1 contains a method for two-phase cooling of heat generating electronic elements. It describes the microporous surface coating of a submerged heater surface with nickel particles of 30 to 50 µm size. The porous surface increases the density of boiling nucleation sites. Coatings with solid particles are also proposed in DE10 2012 108 602 A1 , wherein coatings with a 10-500 um thickness are shown, wherein the coating is made out of sand fixed to the surface with a polymer binder. The sand is an aggregate of solid particles of mineral origin as given in EN 12620 and EN 13139 with D50 ≥ 300 µm particle size.
Further, in Rahman et al., "Increasing Boiling Heat Transfer using Low Conductivity Materials", Scientific Reports 5, 13145, 2015 a micro-structured matrix of insulating material is embedded on the surface of a good thermal conductor, which yields different surface temperatures during nucleate boiling and distinct areas of liquid and vapour flows. The micro-convection and the bubble dynamics increase the nucleate boiling heat transfer.
Also, influence of surface treatments on the heat flux from copper samples to liquid nitrogen is investigated in Hellmann et al., "Influence of Different Surface Treatments on the Heat Flux from Solids to Liquid Nitrogen", IEEE Transactions on Applied Superconductivity, 24(3), 1-5, 2014. The cool-down of samples from room-temperature to liquid nitrogen temperature (78 K) show an improved cooling with Kapton coating. The effect is explained by the lower thermal conductivity of Kapton, which reduces the surface temperature and yields a better boiling heat transfer regime to the liquid nitrogen in this transient cool-down process.
The known heat transfer enhancement techniques of the state of the art in case of heat transfer to single-phase flow have the general disadvantage of an increased pressure drop, which foils the energetic benefit of heat transfer enhancement particularly in case of gas flow.
Based on the state of the art mentioned above, it is objective of the present invention to provide an improved heat exchanger element, which has an enhanced heat transfer capability and where the pressure drop is not increased.
This object is solved by a heat exchanger element with the features of claim 1.
Another object of this invention, to provide a cost-effective method for manufacturing a heat exchanger element with enhanced heat transfer capability is solved by the method with the features of claim 6.
Further, advantageous modifications of the heat exchanger element and the method are subject matter of the dependent claims.
A heat exchanger element for being in contact with a gas according to the invention comprises one or more solid surface(s). Meaning as such, that a heat exchanger element can have areas, which are coated and areas without any coating. At least one of these areas, one which is in contact with the heat exchanger fluid, here a gas, can be coated. According to the invention, said solid surface is coated with one or more layer(s) of predetermined material, the layer being suitable to enhance the heat transfer between the solid surface and said fluid by thermo-acoustic impedance matching.
This layer is a homogeneous material layer, whereas such a consistent material layer works for a single-phase fluid, such as any gas, effectively. The gas as the heat exchanging fluid can be a pure gas, a mixture of gases or an aerosol, i.e. a suspension of particles and/or droplets in a gaseous phase. The physical basis for the layer can be explained as follows:

Classical heat transfer theory is based on the concept of a thermal (as well as hydrodynamic) boundary layer, whereby the temperature of the fluid in contact with the wall is equal to the wall temperature and a decline in temperature occurs as Fig. 1a shows.

A fundamentally different behaviour was discovered by P. L. Kapitza ("The study of Heat Transfer on Helium II", Journal of Physics (USSR) 4, 181, 1941), who found a temperature step during heat transfer from a solid wall to the quantum fluid Helium II, as shown in Fig. 1b . This effect is generally referred to as "Kapitza conductance" or "Kapitza resistance", respectively. The physical mechanism is explained by various forms of the acoustic mismatch model (AMM) of phonon propagation. A specific model for the thermal resistance between Helium II and different metals is described in Budaev et al. "A new acoustic mismatch theory for Kapitza resistance", Journal of Physics A-Mathematical and Theoretical, 43(42), 2010. Experimental data for the Kapitza resistance in Helium II systems with Kapton sheets of 14-130 m thickness is disclosed in Baudouy et al. "Kaptiza resistance and thermal conductivity of Kapton in superfluid helium", Cryogenics, 43 (12), 2003.
As a general consequence of the Kapitza resistance, the differences in both the thermo-acoustic impedances and other thermal properties of the solid and the gas are responsible for the thermal resistance and thus the temperature step at the interface.
The layers of predetermined materials and the layer thicknesses are chosen such that the heat transfer between the solid surface and the gas is enhanced by thermo-acoustic impedance matching between all the layers in contact with each other. With this implementation, the sum of thermal resistances and thus the overall temperature difference between the solid surface and the gas is smaller than in case of the uncoated solid surface. Candidates for the coating and therefore examples for the predetermined materials are materials with intermediate values of thermo-acoustic impedance. This promotes in particular non-metallic amorphous materials rather than crystalline materials, as the latter have impedances in the range of metals or beyond.
Further, another embodiment of the invention can implement, that said solid surface has at least one flat section or at least one section with a predetermined structuring or topology or a combination of both. The heat transfer enhancement according to the invention can be applied on either side of said solid surface. Further, it can be combined with the other enhancement methods, in particular with surface enhancement methods.
The heat exchanger element and or its surface can be made out of any suitable solid, such as copper, aluminum, steel, silicon, graphene or diamond, for example.
In another preferred embodiment of the invention, said solid surface has either at least one section which has a tubular shape with an outer surface and/or an inner surface both with a curvature, either concave or convex, as for example a tube segment, or which has at least one section which has a cylindrical shape with an outer surface and/or an inner surface, as for example a cylindrical body with a bore. The heat transfer enhancement according to the invention can be applied on either side of said solid surface. Further, it can be combined with the other enhancement methods, in particular with surface enhancement methods. The heat exchanger element and or its surface can be made out of any suitable solid, such as copper, aluminum, steel, silicon, graphene or diamond, for example.
Heat transfer enhancement between solid surfaces and gases can be applied by means of thermo-acoustic impedance matching. Thermo-acoustic impedances depend on the speed of sound and the mass density. The values for solids and gases can differ by several orders of magnitude. The layer materials and the layer thicknesses might be chosen such that the heat transfer between the solid surface and the gas is enhanced by thermo-acoustic impedance matching between all the layers in contact with each other. If the layer is too thin for the phonon excitation, there will be no effect of the layer, if it is too thick, said layer will function as a thermal insulator.
With this, the sum of thermal resistances and thus the overall temperature difference between the solid surface and the gas is smaller than in case of the uncoated solid surface. This is counter-intuitive with regard to classical heat transfer theory, where such coating would act as thermal insulation and worsen the heat transfer due to the additional thermal resistance(s) of the layer(s). Candidates for the coating are materials with intermediate values of thermo-acoustic impedance. This promotes in particular non-metallic amorphous materials rather than crystalline materials, as the latter have impedances in the range of metals or beyond.

The values for solids, liquids and gases differ by orders of magnitude as shown by the example data in Tab. 1.

Tab. 1: Thermo-acoustic properties of selected materials.

Material	Density / kg/m³	Longitudinal wave velocity / m/s	Impedance / Pa s/m
Helium @ 293K	0,166	1010	1,7E+02
Nitrogen @
293K	1,17	349	4,0E+02
LNG @ 77K	808	855	6,9E+05
Water @ 293K	998	1480	1,5E+06
LD-PE	920	1950	1,8E+06
Polyurethane	1110	1760	2,0E+06
Glas: pyrex	2240	5640	1,3E+07
Aluminum	2700	6420	1,7E+07
Steel	7800	5850	4,6E+07
Copper	8930	5010	4,5E+07

For instance, the implementation of a LD-PE coating on a steel surface would enhance the heat transfer to gaseous nitrogen (or air) at 293 K, because the impedance (1.8E+06) is in-between that of steel (4.6E+07) and that of nitrogen gas (4.0E+02). On the other hand, the same LD-PE coating would have a negligible or even negative effect in case of heat transfer to water at 293 K, because the impedances of water (1.5E+06) and LD-PE are nearly the same and the LD-PE layer would thus not improve the thermo-acoustic impedance matching and only act as an additional thermal insulator. Depending for which gas the heat transfer has to be enhanced, the best layer-coating can be chosen and used. The effect is very effective for the heat transfer enhancement between solids and gases, which show the largest mismatch in thermo-acoustic impedance.
Due to the mechanism of phonon propagation, the thickness(es) of the coating layer(s) is one design parameter for thermo-acoustic impedance matching, beside the choice of the layer material. Preferably, said layer can have a thickness in a range between 1 µm to 100 µm. Thinner layers of non-matching material might therefore be applied on the surface next to the gas, without disturbing the thermo-acoustic impedance matching. An example are sub-micron metallic layers for UV or corrosion protection, for the prevention of fouling or for optical reasons. The thickness can be adapted according to phonon propagation properties of specific materials.
In a further embodiment, said solid surface can be coated with several layers out of different predetermined materials and having different thicknesses.
In a preferred embodiment, said heat exchanger element can be manufactured according to the method described according to the invention. This enables a simple and cost-efficient manufacturing of an enhanced heat exchanger element. For being in contact with a gas, said heat exchanger element comprises one or more solid surface(s). The manufacturing comprises the step of coating said solid surface with one or more layer(s) of predetermined material, wherein said layer is suitable to enhance the heat transfer between the solid surface and said gas by thermo-acoustic impedance matching.
In another embodiment of the invention, coating of said layer is performed onto the solid surface by slot-die coating, doctor blading, dip coating, spray painting or alternatively by the lamination of films. These methods can be used continuously (roll-to-roll).
Further embodiments and some of the advantages, which are connected to these and other embodiments, are made comprehensible in the following description by reference to the figures. Said figures are a schematic representation of an embodiment of the invention.
It is shown in

Fig. 2: a schematic side view of a part of a heat exchanger element with one layer;
Fig. 3: a heat exchanger element with more than one layer; and
Fig. 4: a heat exchanger element with two coated surfaces.

In Fig. 2 a heat exchanger element 1 (shown as a part) has a solid surface 2. Said heat exchanger element 1 is in contact with a gas 3 serving as a heat exchanger medium, which might be any form of gas.
Onto the solid surface 2 a layer 4 is coated, so that the layer 4 is part of the heat exchanger element 1.
The heat transfer direction is represented by arrow 5 and the generated temperature profile is depicted by curve 6. The temperature profile results from the consideration of both Kapitza conductance and thermal boundary theory in case of a gas 3 flowing along a heat exchanger element 1. The total temperature difference results from the first and second temperature steps 6a, 6b, a temperature gradient 6c and a thermal boundary layer 6d (shown by curve 6). Due to thermo-acoustic impedance matching, the total temperature difference 7b between the surface 2 and the gas 3 is smaller than the temperature difference of the uncoated wall, which would result in a larger temperature difference 7a (indicated by the dashed lines). As the layer 4 does not influence the thermal boundary layer by the promotion of turbulence, the heat transfer enhancement is due to thermo-acoustic impedance matching.
In Fig. 3 three layers 4, 4', 4" of predetermined material are coated onto the surface 2 in order to gain a step-wise thermo-acoustic impedance matching. With multiple layers 4, 4', 4" applied on the surface 2, the thicknesses and materials of the layers 4, 4', 4" might be chosen such that the transition of the thermo-acoustic properties from the surface 2 to the gas 3 is smoother than for a single coating layer 4 ( Fig. 2 ). This leads to further reduction in the total temperature difference 7c compared to the temperature difference 7b of a single layer, as curve 6 depicts.
Further, in Fig. 4 the heat exchanger element 1 has two metallic surfaces 2, 2' which both have a layer 4, 4' as a coating. Such a heat exchanger element 1 can be used in order to specially adapt each surface 2, 2' with customized thermo-acoustic properties. Arrow 5 in Fig. 4 illustrates the transfer of a heat flux from a gas 3 to a gas 3', both being separated by a solid heat exchanger element 1. In this case, the properties of the layers 4, 4' on either side of the surfaces 2, 2' of the heat exchanger element 1 are designed to match the respective gas 3, 3'. This leads to reduced temperature steps on either side, resulting in a lower total temperature difference (see temperature profile 6).

LIST OF REFERENCE NUMERALS

1: Heat exchanger element
2: Solid surface
3,3': Gas
4, 4', 4": Layer
5: Heat flux
6: Temperature profile
6a, 6a': First temperature step
6b, 6b': Second temperature step
6c, 6c': Temperature gradient in the layer
6d, 6d': Temperature gradient in the boundary layer
7: Overall temperature difference
7a: Temperature difference of an uncoated surface
7b: Temperature difference of a surface coated with one layer
7c: Temperature difference of a surface coated with more than one layer

Claims

Heat exchanger element (1) for being in contact with a gas (3),
wherein the heat exchanger element (1) comprises at least one solid surface (2),
characterized in that
said solid surface (2) is coated with at least one layer (4) of a predetermined material, said layer being suitable to enhance the heat transfer between the solid surface (2) and said gas (3) by thermo-acoustic impedance matching.
Heat exchanger element (1) according to claim 1,
wherein
said predetermined material of said layer (4) is a material with values of thermo-acoustic impedance between the values of said heat exchanger element (1) and said gas (3).
Heat exchanger element (1) according to claim 1 or 2,
wherein
said at least one layer (4) has a thickness in a range between 1 µm to 100 µm.
Heat exchanger element (1) according to one of the claims 1 to 3,
wherein
said solid surface (2) has at least one flat section or at least one section with a predetermined structuring and/or topology or a combination of both.
Heat exchanger element (1) according to one of the claims 1 to 4,
wherein
said solid surface (2) has at least one section which has a tubular shape with an outer surface and/or an inner surface and/or a cylindrical shape with an outer surface and/or an inner surface.
Method for coating a heat exchanger element (1) being in contact with a gas (3),
wherein the heat exchanger element (1) comprises at least one solid surface (2),
comprising the step of
coating said solid surface (2) with at least one layer (4) of a predetermined material,
wherein said layer (4) is suitable to enhance the heat transfer between the solid surface (2) and said gas (3) by thermo-acoustic impedance matching.
Method according to claim 6,
wherein
said predetermined material is a non-crystalline material having intermediate values of thermo-acoustic impedance, such as non-metallic amorphous materials.
Method according to claim 6 or 7,
wherein
coating of said layer (4) is performed onto the solid surface (2) by slot-die coating, doctor blading, dip coating, spray painting or by the lamination of films.
Method according to one of the claims 6 to 8,
wherein
said solid surface (2) is coated by several layers (4) out of different predetermined materials and/or having different thicknesses.