Emerging Technologies for Heat Exchanger Application*
Due to the innovations brought about by heat transfer equipment designers, the increased availability of advanced modeling technologies and the applicability of advanced materials, the field of heat exchanger research and development has and is making large strides. These strides lead to a proliferation of options for the design engineer that require a fundamental understanding of the physics involved, an adaptation of materials and configurations, a common experimental baseline and an ability to analytically examine all of the variables associated with both heat exchanger design and its integration into a higher level system. The later of these considerations, that of analytical modeling for heat transfer equipment, is one that involves numerous variables and exits in tightly constrained, multi-dimensional space. These are the parameters that define and resulted in the study of multi-variable optimization.
Due to recent advances in all aspects of materials development many new materials that provide high thermal conductivity, high strength to weight ratios, and wide temperature range of applicability have come to prevalence. Case studies have shown that increasing the thermal conductivity in a heat exchanger provides weight and volume reduction benefits until the thermal conductivity approaches that of aluminum or copper. Some of the advanced materials considered for enhancement research are the following: Carbon-Carbon: C-C has high thermal conductivity, low density, and high strength to weight. It can be used for moderate and high temperature applications. High temperature applications however require the use of oxidation inhibitors or coatings. Ceramic: Research on the use of ceramics for heat exchangers have resulted in high temperature, high effectiveness, low-pressure loss, and low weight heat exchangers. Titanium Alloy & Titanium Matrix Composites (TMC): Titanium has high thermal conductivity, a density that is about 60% of stainless steel and good corrosion resistance. It also combines well with other metals such as aluminum, vanadium, zirconium, tin, and iron to create TMCs. Titanium is an intermediate solution for the high temperature heat exchanger regime. Once carbon-carbon technologies are developed, titanium will be obsolete. Aluminum Alloy & Aluminum Matrix: Aluminum has been widely used in heat exchangers. It has high thermal conductivity and a 65 % lower density than stainless steel. Aluminum is limited to low and moderate temperature applications due to its low melting point. Beryllium Alloys: Beryllium Alloys are 22% lower in density, has a lower coefficient of thermal expansion and a high thermal conductivity than aluminum.
Utilization of these advance materials requires some insight into their range of applicability. Once that has been determined, cost, specific conductivity, working fluid interactions, mechanical properties, among other variables must be considered. Additionally, because these materials have become so abundant in all aspects of mechanical system design, designers should consider the integration of heat transfer equipment and structural element. The fruition of which will revolutionize system architecture.
The use of advanced materials is just one variable in the optimization of heat transfer equipment. Heat transfer enhancement technologies must also be considered. These technologies can be divided into two basic classes, passive and active enhancements. Passive enhancements require no power supplies, wave generators or other equipment external to the heat exchanger whereas active enhancement techniques may require some interaction with exterior devices. The passive techniques can be further categorized as those that result from physical configuration variations and those that create flow variations. The first of which includes: Extended Surfaces &emdash; Dimpling of fins can be used to improve the overall heat transfer coefficient of the heat exchanger. Modern louvered fin surfaces are strips of metal formed out of the plane of the fin. The fins tend to act as multiple flat plates and can result in enhancements of 2-3 times that of a plane-fin geometry. Parallel Tubes &emdash; Parallel thin walled flat tubes with internal laminar flow provide a high heat transfer to pressure drop ratio and high heat exchanger effectiveness. A heat exchanger with parallel thin flat tubes should approach the performance of laminar flow between two flat parallel plates. A spiral plate heat exchanger when used in conjunction with other enhancement techniques may also provide a further compact design. Micro Channel &emdash; Micro channel heat exchangers are characterized by numerous miniature channels and varied fin arrangement. Micro channel heat exchangers have characteristic length scale on the order of hundreds of microns. They allow for high heat loads, low temperature differences and low thermal resistance.
Other passive enhancement techniques involve disturbance of the flow field or create local variations in the flow field to increase the heat transfer coefficient. Some examples of this type of enhancement include aspirating, swirling and injection or suction devices. Aspiration can be used in is either parallel or counter flow direction. The flow in a narrower portion of the channel has a higher velocity and a lower pressure than the flow in the nominal channel. The opposite is true in the adjacent channel. When the walls are perforated, there will be an enhancing cross-flow of fluid. This disturbs the axial flow which elevates the heat transfer coefficient. Swirl Flow Devices or vortex generators can be incorporated into the heat transfer surfaces of plate-fin, tube-and-plate fin, or finned-tube heat exchangers. The generators could be stamped, punched or embossed. Secondary flows due to this device elevate the heat transfer coefficient. Increases in pressure drop will be in proportion to the increase in heat transfer. Injection and suction utilize a supply or removal of a gas through a porous heat transfer surface. The net effect is a reduced boundary layer or film resistance. This enhancement technique is applicable to both single and two phased flows. Additional power is required to force the gas supply or to suction the gas through the porous surface.
Recent advances in active heat transfer enhancement have made it feasible to design "on-demand" heat exchangers. These heat exchangers are designed to operate at nominal load without the enhancement being used. Once the load deviates from nominal, an integrated control system activates the enhancement mechanism to increase the capabilities of the heat exchanger. Some of the most common means of active enhancement use ultrasonic generators or electric fields to create secondary flows in heat exchanger fluid passages. One example is called fluid vibration. Fluid vibration uses ultrasonic frequencies in the range from 1 Hz to provide heat transfer enhancement in single-phase heat exchangers. Vibration is applied to the fluid via a flow interrupter or an electric transducer. A power source along with electrical controls is required to produce the heat transfer enhancement. Another means of actively creating secondary flows is electrohydrodynamics. This system uses high voltages introduced via electrodes in a dielectric flow stream. Secondary flows caused by electrophoretic or dielectrophoretic forces create large enhancements to heat transfer. A power source along with electrical controls must be supplied in order to produce the heat transfer enhancement. The final active enhancement technique is derived from the passive swirl flow device concept. It has an active vortex generator and may allow for an on-demand type cooling arrangements.
This article is not intended to be all inclusive, in that all advances in heat exchanger development cannot be examined in this format, however, it was intended to allow the presentation of some thoughts and a possible views of near and far term developments.
* This article was origincally published at ASME PID (Process Industry Division) Newsletter, Spring 1999
Note: A short Course on Heat Exchangers: Emerging technologies in HXs and HX design, Contact ASME PID Chairman, Michael Ohadi at ohadi@eng.umd.edu or David M Pratt at David.pratt@va.afrl.af.mil