Discussion on heat transfer in microchannels

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Everyone is currently paying more attention to the heat transfer issues within microchannels. As is well known, the greatest feature of microchannel chemical equipment is its strong heat transfer capacity. However, there are few manufacturers and research institutions that have conducted systematic summaries on the factors affecting the heat transfer performance of microchannel heat exchangers and reactors. The author previously conducted dedicated research on the heat transfer coefficient of microchannels, and here I will only share some of the results. The figure shows the heat transfer performance of microchannels under different characteristic heat transfer scales, for two materials and three channel shapes. This diagram can provide answers to many issues related to microchannel heat transfer. It can also provide some guiding ideas for the design of microchannels. It should be clear first that the main factor affecting the heat transfer coefficient in microchannels is the characteristic size of heat transfer; within the range of 0.1–5 mm, this size has a significant impact on the heat transfer coefficient of microchannels. This is also a design criterion that needs to be given priority in the design process. Next is the impact of materials; currently, the materials used in microchemical modules include metals (stainless steel, titanium, Hastelloy, etc.) as well as ceramic materials. For systems with low corrosion resistance requirements, such as micro-heat exchangers, brass with a higher thermal conductivity can be considered. However, purely in terms of heat transfer capacity, for flow channels with characteristic heat transfer dimensions greater than 1 mm, the difference in heat transfer coefficients between different materials is no more than 20%; the thermal conductivity of the material itself has little impact on the system’s heat transfer capacity. Conversely, the smaller the channel scale, the more significant the influence of the material becomes; for microchannels below 0.5 mm, it is necessary to consider appropriate material selection for the equipment in order to maximize the thermal conductivity. Thirdly, the complex microchannel geometry helps improve heat transfer efficiency, with an improvement factor of approximately 1.2–1.4 times that of simple channels. For flow channels with characteristic dimensions greater than 1 mm, the difference in the material’s heat transfer capacity can be fully compensated for by altering the shape of the flow channel. None of the above graphs take the effect of flow regime into account; in fact, they represent experimental results under laminar flow conditions. The heat transfer capacity further increases when turbulence develops within the flow channel. However, this situation requires a high flow rate, which is relatively rare in actual research and development. A current approach in channel design is to maximize fluid disturbance, thereby causing the flow to become turbulent at low Re numbers. Of course, the heat transfer capacity of a device is influenced by many parameters, with the overall heat transfer coefficient being just one small part of them. The specific surface area of the equipment, the temperature of the refrigerant under the process conditions, and the physical properties of the fluid all affect the final heat transfer efficiency. For the design of micro-heat exchange systems, the author has identified a general set of design principles: “1. In terms of material selection, priority should be given to materials with good corrosion resistance; provided that such corrosion resistance is sufficient, materials that are inexpensive and easy to process should be chosen.” 2. On the basis of materials being easy to process, efforts can be made to minimize processing costs while reducing the dimensions related to heat transfer characteristics. It effectively compensates for the insufficient thermal conductivity of the material itself. 3. Use reinforced flow channels as appropriate, but avoid overcomplicating them, as this will increase processing costs and be counterproductive.