BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a heat exchanger which uses a drag reducing fluid as its heat transfer medium. Heat exchangers of this type are used in district heating/cooling systems, heat transportation systems for heating and cooling, andindustrial processes that employ hot water as a heat transfer medium in the production of chemical products, processed foods and the like.

2. Description of the Prior Art

The drag that arises between the surface of the heat exchanger and the heat transfer medium with which it exchanges heat is known to increase the amount of pumping power required for circulating the heat transfer media. When a heat transfermedium such as hot water is pumped through a long pipeline in a district heating system, for example, the system is required to have very large pumping power to overcome the drag between the heat transfer medium and the pipe wall. Since a reduction indrag translates into a reduction in the required pumping power, it has been proposed to use a drag reducing fluid (DR fluid) as the heat transfer medium. It is known that a DR fluid, which is water mixed with a surfactant or a water-soluble polymer, canreduce drag in a pipeline to about one fifth that of water alone. From this it follows that the pumping power cost which accounts for 30% of the operating expense of a district heating system could be reduced to about one fifth by use of a DR fluid asthe heat transfer medium. (Bewesdorff, H. W., Structure of Turbulence and Drag Reduction, (1990), 293, Springer et al.)

It is widely known, however, that the drag reduction obtained by use of a DR fluid comes at the expense of a loss in the heat exchange performance of the heat exchanger. Application of DR fluid-based drag reduction technology to a districtheating system therefore requires installation of a large and complicatedly configured heat exchanger. As a result, the initial investment is large and the reduction in pumping power is not as great as might be hoped.

This invention was accomplished in light of the foregoing circumstances and has as its object to provide a heat exchanger which utilizes the special properties of a DR fluid to achieve small drag while also achieving high heat exchangeperformance in the heat exchanger.

SUMMARY OF THE INVENTION

For realizing this object, the present invention provides a heat exchanger using a drag reducing fluid wherein the heat transfer surface of the fluid passage on the drag reducing fluid side is formed with irregularities.

The physical properties a DR fluid in a heat exchanger using a DR fluid vary in sharp response to changes in temperature. By providing the heat transfer surface of the heat exchanger with appropriately shaped irregularities for utilizing thischaracteristic of the DR fluid, the heat transfer of the heat exchanger can be increased while taking advantage of the drag reducing effect of the DR fluid.

As background information for making the invention easier to understand, a brief explanation will be given regarding the viscosity of a DR fluid and the friction drag that appears during laminar and turbulent flow.

Turbulent flow is characterized by the presence of numerous eddies moving randomly in the flow field and is commonly observed in high Reynolds number flow such as occurs, for example, in rivers, the wind and industrial facilities. Laminar flowappears in the low Reynolds number region and is seen, for example, in capillaries and viscometers. In laminar flow, there is no random eddy motion.

Drag reducing (DR) fluids reduce drag caused by turbulent flow, which is by far the most common kind of flow encountered in industry. Because of this, the reduction of drag in turbulent flow is usually referred to simply as "drag reduction,"although the precise term is "drag reduction in turbulent flow." This abbreviated usage is adopted in this specification.

A DR fluid is usually composed of a solvent and a drag reducing additive. In turbulent flow, the DR fluid exhibits lower drag than the solvent under the same conditions. In laminar flow, it exhibits higher drag. These completely oppositeeffects in different flow regimes is thought to result from the presence of the drag reducing additive in the solvent in the form of "large-scale structures."

A typical DR fluid consists of water (the solvent) and a polymer or

surfactant (the drag reducing additive). The apparent viscosity of such a DR fluid, as determined by viscometric measurement, is higher than that of its solvent alone. This is because in laminar flow the large-scale structures formed by theadditive increase the apparent viscosity and produce greater drag than the solvent alone flowing at the same rate in a pipe of the same diameter. In other words, in the simple fluid motion appearing in the laminar state, the fluid property directlyaffects the drag in flow.

The relationship between viscosity and drag is very different in turbulent flow. In turbulent flow, there are numerous eddies or vortices that are effective carriers of momentum and heat and the turbulent mixing they cause produces largefriction drag and increased heat transfer.

Since the friction drag caused by the random motion of eddies in turbulent flow is several tens of times greater than that occurring in laminar flow, viscosity does not significantly affect drag in high Reynolds number turbulent flow.

The widely accepted explanation for this is that the large scale structures formed by the additive in the DR fluid suppress the eddies occurring in turbulent state, thus drastically reducing turbulent drag. One theory holds that the turbulenteddy suppression is the result of enhanced fluid elasticity or elongation viscosity in the DR fluid. At any rate, it is known that these properties increase with shear viscosity measured in the laminar state. Therefore, increased shear viscositybecomes a measure of the formation of large scale structures in the DR fluid, i.e., a measure of drag reduction effect in the turbulent flow regime.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is an enlarged schematic diagram of an essential portion of a heat exchanger according to the invention.

FIG. 2 is a graph showing an example of the temperature effect on the physical property of a DR fluid.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will now be explained in detail with reference to an embodiment shown in the drawings.

In FIG. 1, reference numeral 1 designates a schematic representation of the heat exchange portion of a heat exchanger. The heat exchange portion 1 is partitioned into two fluid passages 3 and 4 by a heat transfer plate 2. A DR fluid 7 is usedas the heat transfer medium flowing through the fluid passage 3. The DR fluid 7 is a solution of a surfactant or a water-soluble polymer in pure water. As the surfactant of the DR fluid there can be used, for example, hexadecyl trimethyl ammoniumchloride (C.sub.19 H.sub.42 NCl, also know as cetyl trimethyl ammonium chloride) produced by Nacalai Tesque Co., Japan. This type of DR fluid is well known and commercially available. The fluid flowing through the fluid passage 4 exchanges heat withthe DR fluid flowing through the fluid passage 3. It can be a liquid such as water or oil but is not limited to a liquid and can instead be a high-temperature gas or other such heat transfer medium.

The heat transfer surface 6 of the heat transfer plate 2 on the side of the fluid passage 3 is formed with irregularities. The shape of the irregularities can be wavy as shown in the drawing but can instead be fin-like or be configured in any ofvarious other shapes.

A specific example of the irregularities of the heat transfer surface 6 would be triangular waves having a pitch of about 23 mm and a wave height of 7.5 mm.

While the shape of the heat transfer surface 5 on the side of the fluid passage 4 is wavy in the illustrated embodiment, it can be of any shape, including flat.

FIG. 2 shows the temperature effect of CTAC (an aqueous surfactant solution) which is suitable for use as the DR fluid in the heat exchanger according to the invention. The vertical axis of the graph of FIG. 2 is scaled for the shear viscosityof the liquid and the horizontal axis for the absolute temperature of the solution. It is widely known that shear viscosity is a measure of the formation large structures in a DR fluid. The drag reduction effect in turbulent flow can therefore beexpected to increase with increasing viscosity. In the vicinity of 270 K, the viscosity of the DR fluid is low and coincides with that of water (point "C"). At around 280 K, however, the viscosity rises sharply. This indicates the formation of largescale structures in the solution and suggest that, in turbulent flow in the heat exchanger, the DR fluid exhibits its DR effect, while its heat transfer coefficient falls simultaneously (point "B"). As the temperature rises further, the viscosity of theDR fluid has a large value and the heat transfer coefficient is thought to be low up to around 315 K (point "D") in turbulent flow. When the temperature exceeds 320 K, the coefficient of viscosity falls rapidly. This change corresponds to thedestruction of the large scale structures in the solution due to high temperature. The physical property of the DR fluid therefore becomes the same as water and the turbulent flow and heat transfer coefficient reduction effect is thought to be lostsimultaneously (point "A").

The operation of the invention heat exchanger when the DR fluid with the aforesaid temperature effect flows through the heat transportation system will now be explained. The system is designed to maintain the DR fluid flowing out and backthrough pipelines of the heat transportation system at a temperature lower than that at point "A" in FIG. 2 (e.g., at a temperature between points "D" and "E"). This maintains the presence of the large scale structures in the DR fluid and reduces theturbulent drag of the DR fluid in the pipelines, thus lowering the main portion of the fluid drag of the heat transportation system. By lowering the heat transfer coefficient, it also minimizes the amount of heat escaping from the pipelines.

The turbulent DR fluid flows into the heat exchanger. At this time, the heat transfer surface 6 of the heat transfer plate 2 is heated to a temperature higher than point "A" by the fluid in the fluid passage 4. As a result, only the DR fluid inthe vicinity of the heat transfer surface 6 loses its DR effect, causing an increase in drag and raising the heat transfer coefficient of the fluid near the heat transfer surface. Since the heat transfer plate 2 is formed with irregularities, thevelocity of the DR fluid is accelerated near protruding portions of the heat transfer surface 6, such as at point a in FIG. 1, as indicated by the arrows. Since the thermal boundary layer becomes thinner in these regions, good transfer of heat isobtained. Moreover, loss of the large scale structures in the solvent causes active motion. This turbulent eddy motion acts to diminish the pressure gradient near the separation point. Therefore, the flow separation point shifts downstream, and thedistance between b and c increases to produce a further rise in the amount of heat exchange.

As is clear from the foregoing, the heat transfer efficiency of the heat exchanger can be enhanced by providing the heat transfer surface 6 with irregularities of an appropriate shape.

The fluid flowing through the fluid passage 4 is for heating the heat transfer surface 6 of the heat transfer plate 2 to a temperature higher than the temperature of point "A" in FIG. 2 and can be water, oil or gas heated to a prescribedtemperature. It is also possible to embed an electric heater or the like in the heat transfer plate 2 and use it to directly heat the heat transfer surface 6 to a temperature higher than point "A."

Fluid heated to a high temperature is present only in a limited region near the heat transfer plate and is therefore small in quantity. When it mixes with the rest of the fluid on exiting from the heat exchanger, therefore, the temperature ofthe fluid as a whole averages to a point below point "A" (e.g., point "E"), whereby the DR effect is reestablished and the drag in the pipeline decreases.

If there is a possibility of heat exchange proceeding more than expected in the heat exchanger so that the temperature of DR fluid rises markedly to the point that no DR effect is obtained in the pipelines, it suffices to use a DR fluid thatmaintains its DR effect even at high temperatures.

Since the heat exchanger according to this invention utilizes the special properties of a DR fluid and has its heat transfer surface provided with irregularities of a special configuration, the drag of the DR fluid increases at the time ofreceiving heat from the heat transfer surface to thereby expand the range of its contact with the heat transfer surface, the heat transfer coefficient increases and raises the heat exchange rate, and the DR effect of the DR fluid is restored afterseparation from the heat transfer surface, whereby the overall heat transportation system achieves both low fluid drag and high heat transfer performance in the heat exchanger.

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