New design of a liquid bridge heat switch to ensure repetitive operation during changes in thermal conditions

Abstract

A liquid bridge heat switch is investigated to ensure proper ON/OFF operation in the presence of a temperature gradient. A temperature gradient along a plate can lead to changes in liquid properties such as the surface tension and contact angle. Eventually, these changes deteriorate the stability of the switching operation. The stationary position of the liquid bridge moves toward the colder zone over repetitive operations, and residuals of the liquid bridge remain after retraction. In addition, the liquid bridge cannot be generated properly with a predetermined clearance that is sufficient to form the liquid bridge between two plates with a uniform temperature. In order to enable a repetitive switching operation, a conical surface is employed at the hot plate of a heat switch just above the liquid channel. The conical surface reduces the clearance between the top plate and the liquid channel. Also, it provides the highest wettability at the desired zone and maintains the stationary position of the liquid bridge. The effect of the conical surface is evaluated with an LED device in terms of cooling time and thermal resistance. The conical surface extends the thermal resistance range more than three times. As a result, a design methodology for the liquid heat switch system is suggested to guarantee a stable switching operation against changes in thermal conditions. Moreover, the cyclic switching operation reduces the cooling time by almost 20 s compared with the non-cyclic operation.