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Description
In order to homogenize the temperature distribution over an electronic box, the use of heat pipes is investigated. The operating temperature range is between + 20 °C and + 85 °C. The expected main working range is between + 20 °C and + 55 °C. The total heat dissipation to be transported is 45 W. It is distributed over three locations: Evaporator 1 (top left position): 7 W; Evaporator 2 (top right position): 10 W; Evaporator 3 (Center position): 28 W. The heat is transported to two condenser areas: Condenser 1 (bottom left) and Condenser 2 (bottom right).
Due to the wide specified operating range, a trade-off between Cu-H2O heat pipes and Al-NH3 heat pipes was performed. As it can be looked up in the figure of merit for working fluids, the optimal operating temperature for Cu-H2O HP is above 50 °C and the optimal working range for Al-NH3 HP is up to 70 °C.
The two different heat pipe types differ in the design. The Cu-H2O Heat Pipe is built as an assembly consisting of four L-shaped heat pipes. One HP connecting Evaporator (Evp) 1 and Evp 3, one connecting Evp 2 and Evp 3, one connecting Evp 3 and Condenser (Cond) 1, and one connecting Evp 3 and Cond 2. All four heat pipes are interconnected via the Evp 3 interface by soldering into an aluminium block. The Al-NH3 Heat Pipe are two separate U-shaped heat pipes. The left heat pipe connects Evp 1, Evp 3, and Cond 1. The right heat pipe connects Evp 2, Evp 3 and Cond 2. No direct contact between the left and the right heat pipe exists.
Temperature measurement along the heat pipes were performed at several operating temperatures (20 °C, 50 °C, 75 °C, and 85 °C). In order not to over predict the performance the measurements were performed with a tilt height of 2 mm against gravity. For the operating temperature of 50 °C, tilt measurements (4 mm, 6 mm, and 76 mm (20 ° inclination)) were performed.
As expected, the performance of the Cu-H2O HP increased with higher temperatures above 50°C. The Al-NH3 HPs performed quite well within the defined temperature range, with a decreasing performance above 70°C. However at 85°C the Al-NH3 HPs still transport the specified power. Furthermore, the gradients along the Al-NH3 HPs at high temperatures are still lower than for the Cu-H2O HP Assembly, which is against expectations.
At a vapour temperature of 20 °C high temperature gradients over the Cu-H2O HP are observed (dT = 12 K at the specified heat load). A significant contributor causing the high gradients are the not ideal working point of water. The gradients decrease with higher operating temperatures and reach their optimum working point at around 75°C (dT = 4.4 K at the specified heat load). Due to the split design of the Cu-H2O HP Assembly, the temperature gradient over the Cu-H2O HP assembly is still higher than the gradient over the single Al-NH3 HP’s (dT = 2.1 K) at the specified applied heat load - even at high operating temperatures. As expected, at a tilt height of 76 mm (20° inclination) no operation of the Al-NH3 HP is possible, whereas the Cu-H2O HP Assembly shows only a marginal performance loss.
