Heat Exchanger Comprising Wave-shaped Fins

Abstract

A heat exchanger for exchanging heat between a first fluid and a second fluid is disclosed; wherein the heat exchanger has improved thermal efficiency and low fluid back pressure. The heat exchanger comprises conduits for conveying the first fluid, wherein the conduits include a plurality of flow passages. The flow passages are defined by a plurality of fins that are continuous along the direction of flow of the first fluid. Each fin includes a wave-shaped region, and adjacent fins sub-divide each flow passage into first and second sections that are interposed by a third section. The wave-shape of the fins creates a continuously varying cross-sectional area for each third section. The variation of the cross-sectional area of the third section, coupled with the wave-shape of the fins, induces a swirl flow between the third section and each of the first and second sections. This swirl flow improves the efficiency of the overall convection heat transfer in each conduit. Further, the overall cross-sectional area of each conduit remains constant even as the cross-sectional areas of individual flow passages changes, which mitigates the development of back pressure in the flow of the first fluid.

Description

FIELD OF THE INVENTION[0001]The present invention relates to energy conversion in general, and, more particularly, to heat exchangers.BACKGROUND OF THE INVENTION[0002]The Earth's oceans are continually heated by the sun and cover nearly 70% of the Earth's surface. The temperature difference between deep and shallow waters contains a vast amount of solar energy that can potentially be harnessed for human use. In fact, it is estimated that the thermal energy contained in the temperature difference between the warm ocean surface waters and deep cold waters within ±10° of the Equator represents a 3 Tera-watt (3×1012 W) resource.[0003]The total energy available is one or two orders of magnitude higher than other ocean-energy options such as wave power, but the small magnitude of the temperature difference makes energy extraction comparatively difficult and expensive, due to low thermal efficiency.[0004]Ocean thermal energy conversion (“OTEC”) is a method for generating electricity which ...

AI Technical Summary

Benefits of technology

[0016]An embodiment of the present invention comprises first conduits for conveying a first fluid through the heat exchanger and second conduits for conveying a second fluid through the heat exchanger, wherein the first fluid and second fluid are thermally coupled by the heat exchanger. The first conduits include flow passages that induce turbulence in the flowing first fluid without significantly increasing fluid back pressure. The turbulence is induced by wave-shaped fins that project into each first conduit to form a plurality of flow passages. The wave-shaped fins are continuous along the direction of fluid flow. Adjacent pairs of wave-shaped fins define three sections in each flow passage: first and second sections that are interposed by a third section. The wave-shape of the fins results in a continuous variation of the cross-sectional area of the third section along the direction of fluid flow. As this cross-sectional area changes, the first fluid flowing through the flow passage is forced to exchange between the third section and each of the two remaining sections of the flow passage. This exchange of fluid between the three sections induces a swirl, or vortex, flow in the first and second sections, which increases the overall convection heat transfer in the flow passage.

[0017]The fins are arranged within each first conduit such alternating fins project into the first conduit from opposite surfaces and so that the wave shapes of adjacent fin pairs are offset by a phase difference. This phase difference leads to a continuously periodic change on the cross-sectional area of the third section along the length of the flow passage. As the cross-sectional area shrinks, first fluid is “squeezed” from each third section into the first and second sections of each flow passage. As the cross-sectional area of the third section increases, first fluid is drawn back into the third section from the other two sections. Further, the wave-shape of the fins defines a shape of the third section that induces the first fluid to swirl as it enters and exits the first and second sections. This swirl flow creates turbulence that enhances heat transfer between the first fluid and walls of the flow passages.

[0018]It is a further aspect of the invention that the first conduits avoid inducing a significant fluidic back pressure while conveying the first fluid through the heat exchanger. Increased back pressure of the fluid is mitigated by the fact that the overall cross-sectional area of the first conduits remains the same even while the cross-sectional areas of individual flow passages within it change. The consistency of overall cross-sectional area of the conduits results from the complimentary nature of adjacent flow passages within them. Specifically, as the cross-sectional area of a first flow passage is shrinking, the cross-sectional area of its adjacent flow passages is increasing by a commensurate amount. As a result, the sum of the cross-sectional areas of all flow passages in a given first conduit remains constant.

Problems solved by technology

The total energy available is one or two orders of magnitude higher than other ocean-energy options such as wave power, but the small magnitude of the temperature difference makes energy extraction comparatively difficult and expensive, due to low thermal efficiency.

This temperature difference generally increases with decreasing latitude (i.e., near the equator, in the tropics), but evaporation prevents the surface temperature from exceeding 27° C. Also, the subsurface water rarely falls below 5° C. Historically, the main technical challenge of OTEC was to generate significant amounts of power, efficiently, from this very small temperature ratio.

OTEC systems have been shown to be technically viable, but the high capital cost of these systems has thwarted commercialization.

Heat exchangers are the second largest contributor to OTEC plant capital cost (the largest is the cost of the offshore moored vessel or platform).

For OTEC systems, while there are many conventional heat-exchanger designs that can be considered, there are, as a practical matter, no good choices.

This drives the size and cost for this type of heat exchanger and reduces its economic viability.

Unfortunately, plate-fin heat exchangers are undesirable for many applications because of high material and fabrication costs.

Second, brazed joints are poorly suited to applications in which corrosive media are used.

In OTEC applications, brazed joints are particularly susceptible to galvanic corrosion when exposed to seawater.

Third, plate-fin heat exchangers are typically characterized by low thermal and / or flow efficiency.

Such designs suffer from varying amounts of fluid flow resistance, and still do not eliminate manufacturing cost and corrosion issues for very large scale assemblies, however.

Fourth, the small passages found in typical plate-fin heat exchangers are prone to biofouling.

Fifth, maintenance, such as refitting, repair, and refurbishment, on plate-fin heat exchangers is challenging due to the difficulty of accessing their internal regions.

The trade-off with this design is that they typically have lower thermal performance due to a boundary layer created in the fluid(s) being used.

This issue has been studied for decades and has been solved for various applications in different ways with limited success.

Fins that are straight along the flow length tend to develop fluid boundary layers that are quite thick, which results in lower values of the heat transfer coefficient.

More complex fin designs that provide disruptions to fluid flow can improve heat transfer; however, complex fin designs suffer from a higher pressure drop through the heat exchanger.

But development of an OTEC heat exchanger that accommodates high flow rates while minimizing pumping parasitic losses and offering long life in the ocean environment remains elusive.

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