Most standard heat pipes are used to transmit heat from one location to another, with very high effective thermal conductivity. In contrast, annular heat pipes, with an internal cavity, are most commonly used to provide a very high degree of isothermality. An annular heat pipe, also known as an Isothermal Furnace Liner (IFL) is shown in Figure 2. All of the interior surfaces are wicked. Bridge wicks connect the inner and outer cylinders of the heat pipe to allow the fluid to return from the inner to the outer cylinder.
During operation, the annular heat pipe is placed inside an electrically heated oven, in either a horizontal or vertical orientation. Such ovens provide non-uniform heat, both axially and radially. As shown in Figure 3, the heat vaporizes the working fluid in the wick against the outside wall. The vapor travels radially and condenses on the wick against the outside wall (as well as the wick around the thermal wells). The liquid then travels back by capillary action to the outside wick through a series of bridge wicks, and then the cycle repeats.
Most heat pipes are designed to transport large amounts of power with a minimal temperature drop. In contrast, temperature uniformity is more important in IFL applications. Most of the power in a high temperature IFL is radiated from the exterior surfaces, with just enough heat transfer to the interior cylinder to replace heat losses. The IFL temperature in the heat pipe, and on the inner cylinder, is very uniform due to the very high evaporation and condensation heat transfer coefficients. Additional vapor evaporates from the outer cylinder wick where the heat flux is higher, however, the evaporation heat transfer coefficient is so high that the temperature difference is minimized. Similarly, a slightly colder patch on the inside cylinder will receive a higher heat flux until it is at the overall temperature. The expected uniformity inside the heat pipe is on the order of mK.
Annular heat pipes are most commonly used in the temperature calibration industry to calibrate primary temperature standards at temperatures up to 1100°C. Thermal wells can be inserted inside the annular heat pipe to directly calibrate equipment, typically with a freeze-point cell; see Figure 2. For calibrating pyrometers, the interior cavity forms an isothermal black body cavity.
When very precise temperature control is also required, the annular heat pipe is incorporated into a Pressure Controlled Heat Pipe. ACT has fabricated a PCHP furnace with dual heat pipes that can operate from 400 to 1100°C, while maintaining better than 3 mK stability. For more information, read our technical paper, A Novel Closed System, Pressure Controlled Heat Pipe Design for High Stability Isothermal Furnace Liner Applications.
Another important application for annular heat pipes is to isothermalize materials processing reactors, for applications such as sintering, annealing, and crystal growing.
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