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Simulation-Based Engineering

Wed, 10/10/2012 - 9:22am
Cathleen Lambertson, Contributing Editor, Comsol Inc.

Simulation-based engineering design helped generate a first physical prototype of a microchannel heat exchanger.

Comsol Figure 1

A Comsol Multiphysics model of the thermal state of the device. All images: Comsol

Intellectual Ventures (IV), Bellevue, Wash., has one of the largest intellectual property portfolios in the world. In 2008, the company launched an invention/prototype laboratory, the Intellectual Ventures Laboratory(IVL), to support the company’s mission of energizing and streamlining an invention economy.

IVL employs broad interdisciplinary teams of physicists, engineers, chemists, biologists, and physicians tasked with discovering, inventing, and developing advanced technology solutions in a variety of fields.

Much of the progress at IVL relies on modeling and simulation simply because experiments can take a long time to accomplish. Modeling and simulation can provide different and unique insights that would not be easily possible by experimentation. Additionally, in more traditional hardware R&D, IVL researchers use modeling and analysis very closely and iteratively with prototyping and direct experimentation to guide the design directions, interpret experimental observations, reduce cycle times, and to maximize understanding.

Almost all of the projects at IVL use some type of modeling and analysis, and a significant portion of them lend themselves to finite element analysis (FEA). Comsol Multiphysics software is used extensively.

Recently, an IVL team used Comsol Multiphysics to model the design and development of a novel microchannel counter-current regenerative heat exchanger (RHX) to thermally process a liquid stream with exceptionally high heat-recapture efficiency (Figure 1).

Comsol Figure 2

Figure 2: A schematic representation of a unit flow loop in the microchannel counter-current RHX device. A small amount of added heat is enough to maintain the temperature profile in the device once steady state is reached, thanks to very high regenerative efficiency.

An RHX is a type of heat exchanger in which the same fluid is both the cooling fluid and the cooled fluid, meaning the hot fluid leaving the system gives up its heat to regenerate (heat up) the fluid returning to the system (Figure 2). RHXs are usually found in high-temperature systems where a portion of the system’s fluid is removed from the main process and then returned in the opposite direction for further processing. Because the fluid removed from the main process contains energy (heat), the heat from the fluid leaving the main system is used to regenerate (reheat) the returning fluid instead of being rejected to an external cooling medium. And since most of the heat energy is reclaimed, the process gives a considerable net savings in energy.

While large RHX systems have been in use for a long time, their smaller microchannel counterparts are a more recent area of interest. For example, the microchannel RHX devices would be useful in modular applications where small quantities of liquid need to be treated without having access to a large infrastructure and/or energy supply. In addition, microchannel RHX is easily scalable in a way that large systems may not be.

Comsol Figure 3

Figure 3: A sparse array of staggered supports limits deformation of the membranes that separate the microchannel counter-current RHX’s channels. A fluid-flow simulation helped calculate the channel width to compensate for increased flow resistance caused by the supports.

After laying out the basic RHX device architecture, Comsol was used as the main analysis tool to investigate the effect of primary design variables on the device performance. The most important performance attributes of interest in the design were the heat-exchange (regenerative) efficiency, which plays into the device power requirement; and the pressure drop/flow rate relationship, which plays into the pumping requirements. Comsol was also used to explore the structural stability of the device in detail (Figure 3) and to help interpret the experimental results after the first physical prototype was built and tested.

In addition, when it comes to making a physical prototype, challenges such as selecting the correct material set to withstand the inherent temperatures and pressures, or choosing prototyping processes to assemble a functional device can arise. IVL explored adhesives, thermal vias, and photolithography techniques and using simulation tools in an integrated manner with prototyping activities.

Utilizing a simulation-based engineering design approach resulted in the first physical prototype working largely as expected: The prototype design of the microchannel RHX system was shown to be capable of thermally treating a water stream by ramping its temperature from room temperature to about 130 C under pressure to prevent boiling, and back to ambient again with close to 98% regenerative energy recapture in a compact, very low-energy thermal treatment device. The concept was proved quickly and the number of subsequent design iterations was minimized.

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