Team FLO: Design of a Mini-Channel Test System

This group had been tasked with designing a mini-channel heat exchanger system to be used in Dr. Terrell\u27s research laboratory. Mini-channels are compact heat exchangers that are widely used in cooling applications, with fluid flow channels that are on the order of millimeters in depth and width. Mini-channels generally provide more cooling than other systems because the fluid covers much more surface area when split into small channels. Research on minichannels focuses on how much heat is transferred from the surface material to the fluid. There was an existing design in Dr. Terrell\u27s laboratory, so the group did not need to start from scratch. Instead, the group needed to improve the previous design model. This previous model did not completely seal the fluid into the channels and had insufficient insulation. The main objective of this design project was to create a mini-channel that has interchangeable flow configurations where the visible fluid passes through the channels without any leakage between the channels, all while being able to measure temperature change throughout the mini-channel and pressure drop across it. The final design was broken into several parts: substrate, housing, heater, viewing screen, and insulation. Each part is tested independently, and then the system is assembled and overall testing is performed. The aluminum minichannel substrate is covered by a clear transparent viewing material in order to have a clear view of the fluid and to allow sealing to prevent leakage. In order to make sure that there is no fluid leakage, the viewing material is attached to the substrate with vacuum grease and four fasteners that exert downward pressure. O-rings are used to seal the interface between the substrate and the housing. The housing is made oftwo separate pieces of delrin, so any length of substrate can be accommodated. Holes are drilled along the substrate so that thermocouples can be placed along the channels. The strip heater is placed along the bottom of the substrate, and the acetal copolymer insulation is placed along the sides and bottom of the substrate. A strip heater was ultimately decided upon because it was small, efficient, and provided the most uniformity in heating. The heater used provided the necessary heat without going over the power limitations. The insulation chosen was acetal copolymer. Extra pipe insulation was placed directly below the heater. Per the constraints, 90% of input power should be transported out via convective heat transfer to the fluid in the channels. Tests run on the substrate with insulation attached show that 98% of the heat goes to the surface of the channels where it will heat the fluid. After using vacuum grease instead of silicone between the viewing material and the substrate, the system did not leak between the channels or out the sides of the substrate. The interface between the housing and substrate did not leak if attached correctly. The thermocouples, pressure taps, and viewing screen are permanently attached to the minichannel substrate in the final design. The only components that needed to be removed in order to remove the minichannel substrate from the housing are the tabs holding the substrate in place and applying pressure. Tests show that the substrate takes an average of 4:21 to remove and replace, which fits under the constraint of five minutes removal time. Research was done to try to predict how the fluid would behave within the channels in various conditions, and using the final design and the Lab View VI and EEs code made by the group, the predictions were tested against actual results. Test results were within the range of predicted values for most of the test results. Most of the design objectives were achieved under the constraints set out in the project charter. However, some objectives relating to measurements and efficiencies either were not met or are oil met under certain conditions. For example, the group was unable to measure the fluid flow rate within the Lab VIEW VI because there are no pins to connect the fluid pump in the lab to the DAQ. In some cases, the heating efficiency is lower than 90%, and this may be due to various liberties of the design-metal screws hold the system together, and these screws and bolts can act as a heat sink to the open air. Finally, some of the components are unsuited for temperatures up to 200 °C. While this could be rectified by using a different material, problems are unexpected because the system is often run far below the melting point of the components. This is not to say that the questionable components-specifically the polycarbonate screen ­should not be changed. However, the system as a whole meets the objectives for Dr. Terrell to perform his research, and because of the method by which the system is constructed, the faulty parts can easily be exchanged for more robust ones