Noncondensible Gas, Mass, and Adverse Tilt Effects on the Start-up of Loop Heat Pipes

Abstract

In recent years, loop heat pipe (LHP) technology has transitioned from a developmental technology to one that is flight ready. The LHP is considered to be more robust than capillary pumped loops (CPL) because the LHP does not require any preconditioning of the system prior to application of the heat load, nor does its performance become unstable in the presence of two-phase fluid in the core of the evaporator. However, both devices have a lower limit on input power: below a certain power, the system may not start properly. The LHP becomes especially susceptible to these low power start-ups following diode operation, intentional shutdown , or very cold conditions. These limits are affected by the presence of adverse tilt, mass on the evaporator, and noncondensible gas in the working fluid. Based on analytical modeling correlated to start-up test data, this paper will describe how the minimum power required to start the loop is increased due to the presence of mass, noncondensible gas, and adverse tilt. The end-product is a methodology for predicting a "safe start" design envelope for a given system and loop design.

Effect of non-condensable gas on startup of a loop thermosyphon

“…Depending on the amount and distribution of NCG in the system, NCG can cause steady-state performance degradation or even the failure of the entire system. In particular, the existence of NCG in the LHP can prolong the startup time and even cause a failed startup at low heat loads [21].…”

State-of-the-Art Experimental Studies on Loop Heat Pipes

“…); 2/ LHP element geometry: vapor groove design, wick characteristics, etc. ; 3/ Heat input power: start-up is usually more reliable and has small temperature overshoot at high input power; 4/ Heat sink temperature; 5/ Thermophysical and chemical properties of the working fluid: effect on the temperature overshoot; 6/ Ambient conditions and thermal insulation of LHP components; 7/ Non-condensed gas (NCG) presence and amount: NCG can influence boiling process in vapor grooves and promote start-up but NCG can also prevent start-up if it is collected in the wick or/and the evaporator wick core (Baumann et al, 1999); 8/ Evaporator mass: high thermal mass may lead to higher overshoot prior to start-up.…”

“…This pressure drop is related to the temperature differential across the wick in accordance with the Clausius-Clapeyron relation (Baumann et al, 1999). With no flow in the system and no heat load on the evaporator, no pressure or temperature gradient will exist across the evaporator wick.…”

Experimental investigation on startup of a novel two-phase cooling loop

“…In operation, the most difficult issues are in the startup of the systems, and it is also the main problem needed to be solved for applications in engineering. For instance, the start-up of LHP is often a complicated task, and it is needed to take into account a lot of factors, such as the initial states of the working fluid across the primary wick in the evaporator, the elements geometry structure, and the ambient conditions of the system [6][7][8][9][10][11]. As a novel cooling technology, the mechanically pumped two-phase cooling loop has shown much predomination characteristics and been designed for many years [12,13], however, it is lack of reports on the experimental investigation about this system in public paper, especially for the study on the processes of startup.…”