Julio Ferreira presents joint paper on LHP for ADAS cooling.

On the 11 July 2022, Julio Ferreira from the University if Michigan presented a paper on using a loop heat pipe for vehicle CPU cooling, analysing the peak performance, partial flooding and dryout regimes. The paper was presented at the ASME Summer Heat Transfer Conference in Philadelphia, USA. The paper was a collaboration between the University of Michigan (UoM), General Motors (GM) and Calyos.

Calyos was responsible for the design and production of the loop heat pipe solution as per the specification created by the UoM and GM. The solutions was designed to cool two 480W processors, transporting the heat over two metres to a remote cold source. The goal of the project was to evaluate the feasibility of using loop heat pipes to cool processors used for Advanced Driver Assistance Systems (ADAS).

The UoM undertook extensive testing of the unit to validate the performance of the system in relation to Calyos' simulations. The UoM found that the final prototype very closely reflected the simulated performance by Calyos, validating the accuracy of Calyos' simulation tools.

People:
Julio Ferreira - University of Michigan
Massoud Kaviany - University of Michigan
Erik Yen - General Motors
Olivier de Laet - Calyos
Vincent Dupont - Calyos
Thomas Nicolle - Calyos

Abstract:

A Loop Heat Pipe (LHP) is assembled and tested for removing up to 400 W from two vehicle CPUs, while keeping their temperature under 95°C and using a 2 m-distanced, 45°C-air cooled condenser, and working fluid R1233zd(E). The capillary evaporator wick uses a liquid artery wick supplying liquid to a sawtooth copper wall evaporator. This novel sawtooth structure extends the evaporation area, spreads the liquid, and allows for the vapor escape space, while minimizing the evaporator resistance. For the application flexibility, it is preferred to place the condenser away from the CPUs. The air-cooled condenser fan-power consumption is also important, influencing the condenser thermal resistance. The loop thermal-hydraulics are analyzed (including the wick effective thermal conductivity, permeability, and maximum capillary pressure) and modeled. These include the role of the liquid accumulation on the evaporator, and the partial, local wick dryout under statistical variation in the microlayer wick thickness. The condenser thermal resistance and pressure drop, the transfer lines pressure drops, and the accumulator volume, which can dominate and limit the overall performance, are addressed in the design selection. The model predictions are compared with the performance of the fabricated LHP under variable thermal load and CPU-condenser separation distance. Good agreements are found, under partial flooding at low thermal load, under partial dryout at high thermal load, as well as under thermal loads with peak evaporator performance. Further improvements realized in the second-generation evaporator wick is expected to raise the peak performance and the maximum thermal load significantly.