Multiple orientations research on heat transfer capabilities of ultra-thin loop heat pipes with various channel configurations

Abstract

Battery thermal management system (BTMS) technology attracted more and more attentions as the new-energy automobile industry has gotten swift and violent progress in a few decades. So far, the widely used technologies include air cooling, liquid cooling and Phase Change Material (PCM) cooling. However, a common problem exists in all used technologies is the complex structures as well as the huge weight and volume, which not only increases the extra energy consuming, but also goes against the development requirement of automobile lighting. In the future, it is required without doubt that the battery thermal management system should be conveniently installed, light in weight and compact in volume, and minimized in secondary energy consumption. Thus, as a passive and effective heat transfer method, the heat pipe technology would be quite match the requirement and able to be further applied in BTMS. In order to cater to the developed requirement of lightweight design in battery thermal management system, ultra-thin loop heat pipe (ULHP) prototypes with only 1.5 mm in thickness were developed. The ultra-thin design was achieved by replacing the traditional capillary core structure with micro channels, which however required extra assistance from the gravity for the ULHP system to operate, and brought about the risk of flow instability at the same time. To fully understand the exact effect of channel configuration on the heat transfer performance of ULHP, and conquer the adverse impact caused by flow boiling in micro channels, two different channel configurations inside the evaporator (parallelogram and trapezoid) were special developed and compared. Their dissimilarities in heat transfer characteristic were studied and compared under multi orientations with experiments, which could be divided into various aspects, the critical work angle, as well as the start-up characteristic, the thermal resistance and the flow instability et al, so as to completely reflect the r respective influences of the parallelogram and trapezoid configurations on the operation and system stability of ULHP. The experiments results showed that both these two ULHP prototypes can work under small angles, meeting the demand of working under multiple orientations. Specially, the parallelogram evaporator configurations showed more superior performance with little gravity assistance by better suppressing the flow instability. This kind of ULHP could not only start up under 15° inclination, but also its average evaporator temperature difference between angles was limited in 4%, the largest difference was less than 10%. Nevertheless, the ULHP with trapezoid configuration couldn't work until the placed angles increased to 30°, while its average evaporator temperature difference between angles and the largest difference for the trapezoid one achieved 7.8% and 20% respectively. The parallelogram configuration made contribution on ensuring the one-way forward circulation of the working fluid inside ULHP, the response of start-up process therefore was faster. The "liquid pool" structure formed in parallelogram configuration also played an important role in decreasing the pressure and temperature fluctuation and easing the flow instability, which not only enhanced the heat transfer capability of ULHP, but also effectively reduced the damage to the battery system. BTMS could work more stably with ULHP whose channel configuration inside the evaporator was in parallelogram.