Edited by:Barb Schmitz
不到十年前,如果你问一个master chef which type of cook stove he preferred, he would have answered gas, without question. Now, however, Miele has provided both professional chefs and at-home cooks with another, better option: the induction cook stove.
Induction cook stoves are known for their efficiency, with over 90% of the energy used going directly into heating the food being prepared, compared to gas or electric stoves, which have a 50% efficiency rate. It was the mission of Miele, a world leader in domestic appliances and commercial machines, to create an induction cook stove that provided better precision and speed than a traditional stove, while still retaining optimal efficiency.
炉灶保持凉爽。实际上，冰块几乎没有熔化，而锅内的水则沸腾。
Researchers at mieletec FH Bielefeld, a joint research laboratory between Miele & Cie. KG and the University of Applied Sciences Bielefeld, Germany, used simulation and the multiphysics approach to improve, verify, and optimize their induction stove designs.
Using field-coupled simulation, Christian Schröder, co-founder and scientific director of mieletec, designed an optimized induction stove that achieved this goal. “We’re very happy with what mieletec has developed,” said Holger Ernst, co-founder and scientific director of mieletec and head of the Innovations Department at Miele. “Mieletec was able to create an induction stove that’s more precise than a gas stove, while still providing better energy efficiency.”
由工作人员科学家Werner Klose（左）和Mikhail Tolstykh（右）进行的测试活动的设置。由于炉灶已被删除，因此您可以看到炉子的内部工作。
A Watched Pot Never Boils—Unless It’s on an Induction Stove
A watched pot never boils. Whoever coined this phrase has probably never used an induction stove. Unlike gas or electric stoves, which can take ten to fifteen minutes to boil a pot of water, an induction stove can bring a cold pot of water to boiling in a matter of minutes. This is achieved using an entirely different heating technique than that of traditional stoves: induction heating.
感应炉会加热放置在炉子表面上的锅。尽管这听起来类似于气或电炉功能的方式，但实际上是一个完全不同的过程。感应炉子加热锅的金属，而不是炉灶。Schröder说：“与其在炉子上进行加热，而不是在炉子上进行加热，实际上是在锅内进行的，使炉子凉爽。”传统炉子加热炉子的表面，然后通过传导加热锅。
The working principle of an induction stove is based on the inductive heating effect. First, a pot is placed on copper coils located underneath the stovetop. When an alternating current (AC) is passed through the copper coils, it generates a magnetic field that induces currents inside the metal of the pot. These induced currents, called eddy currents, heat the pot by the Joule heating effect. The contents of the pot are then heated through conduction first and then convection.
这些图像显示了特殊线圈设计的磁通量密度的X分量。测量（左）和模拟（右）之间有一个很好的一致性。
Initially, induction stoves were designed using a trial and error process, where researchers relied on their experience for an initial estimation of what the frequency, coil size and power output should be. The results were then altered until the best performance was seen, and this became the final design. However, this process can be very expensive and time consuming, and the engineer is left without enough information to know what is happening in the system, and whether the process has truly been optimized.
Schröder说：“模拟使您可以提取您永远无法从实验中获得的数据。”“通过模拟，您可以更好地了解线圈或锅中发生的事情，以便您知道需要优化的是什么。”
Schröder和他的研究人员使用Miele的Comsol Multiphysics和CAD几何形状，能够找到炉灶设计的最佳条件。模拟的准确性使他们能够在Comsol多物理学中优化其结果，因此当构建第一个原型时，他们已经对如何执行有一个清晰的了解。
Design Challenges—When Pots are Singing and Moving on the Stove
线圈诱导的磁场产生了一些有趣的设计挑战。Schröder说：“我们面临的最大问题之一是，当涡流流过它们时，锅会发出的噪音。”“电流产生的高噪声很难摆脱。消除噪声的唯一方法是调整线圈的交替电流和几何形状的频率，以使产生的噪声比人耳的螺距更高。”
“Using simulation, we found the recipe for current frequency and coil design that produced the least amount of noise possible,” commented Schröder. By increasing the frequency to 30 kHz, and optimizing the coil, mieletec built an induction stove that is almost silent to the human ear. Why almost silent? Because, even if the working frequency is not in the audible range, different noise generating phenomena, like magnetostriction, can still occur at lower, audible frequencies.
The comparison between COMSOL Multiphysics results (magnetic flux density norm) and experimental field lines shows that simulation is accurate and can be used to test other coil designs before building a prototype.
The induction heating process was simulated using COMSOL Multiphysics in combination with the AC/DC Module and Heat Transfer Module. In order to obtain an accurate description of what happens in the stove and pot, heat transfer was solved simultaneously with electromagnetics and optimized to determine the best operating conditions.
Another challenge occurs because of the magnetic forces generated by the system. Because induction cookware is made of a paramagnetic metal, its interaction with the magnetic field results in an attraction force that can cause the pot to move around when placed on the stovetop. This occurs because the eddy currents in the pot generate a magnetic field that interacts with the one generated by the coils. If the magnetic forces do not cancel each other out, the resulting net force will cause the pot to move across the stove.
Schröder说：“我们需要知道尺寸以制造线圈，什么形状和使用什么材料。”“这一切都归结为材料科学。”使用模拟，Schröder和他的团队能够了解不同材料的性质如何影响其在热和电磁性能方面的设计，这使他们能够创建一个线圈设计，以确保锅保持pot，同时仍然提供。适量的烹饪功率。
将模拟与原型进行比较
When it came to testing their prototypes, mieletec was impressed with the results that they found. Because their simulations accurately demonstrated how the prototypes would perform, mieletec was able to save a lot of development time and reduce the number of experiments needed to finalize their designs by 80%.
In order to test that their induction stoves heat food homogenously, mieletec uses a pancake to assess heat distribution across the surface of the pan. “If the standard pancake comes out burnt in some places, and undercooked in others, then you know the stove isn’t heating effectively,” Schröder said. “Stiftung Warentest (www.test.de), a trusted German authority on consumer goods, ranked Miele stoves as the best in energy efficiency and more importantly, able to prepare a delicious, nicely browned pancake.”
Simulation of Traditional Appliances
Mieletec is also working on additional kitchen appliances, such as convection ovens. In this application, a complete understanding of the materials and multiphysics interactions taking place were once again important when developing an optimized oven. In order to verify the optimized design, a test is performed in which a porous stone saturated with water is placed inside the oven. Over several days, the stone is passed through various heating cycles to evaluate the oven’s heating rate and energy efficiency.
“We had to work with a natural convection heat transfer simulation, where porous media flow was also involved, a truly multiphysics application,” commented Schröder. “The simulation allowed us to measure the temperature of the stone at points in the experiment that were inaccessible to real sensors,” he continued. “A test that in practice lasts for a few days took only a few hours to achieve when simulated.”
家用电器的未来
Thanks to simulation, mieletec is delivering designs from which consumers are already benefitting in terms of improved energy efficiency and a better cooking experience. The next step is to reach a deeper understanding of the whole induction heating system by including complex multiphysics effects, such as magnetostriction and aging of materials.
Additionally, mieletec is working on the next generation of induction stoves, known as multicoil induction stoves, which will allow a consumer to place their pots and pans anywhere on the stovetop, instead of inside predefined boundaries. The stove will automatically be able to sense the location of the pot, and adjust accordingly to produce optimal cooking conditions by turning on a certain set of coils.
Comsol Inc.
www.comsol.com
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