Smart pumpless loop for micro-channel electronic cooling using flat and enhanced surfaces

Abstract

Two-phase cooling of a square simulated electronic device surface of 21.3 mm side was successfully carried out without the need for a pump. This smart, passive cooling system incorporates a self-enhancing and self-sustaining mechanism, wherein the system inherently enhances its cooling capacity by increasing the velocity of the two-phase mixture along the boiling surface when an increase in heat flux is sensed. Other practical attributes of this pumpless loop are small liquid inventory requirements and absence of the incipient boiling temperature drop. It is shown small surface tension and contact angle render dielectric coolants such as FC-72 ideally suited for flow in narrow gaps. These unique properties are responsible for very small bubble size, precluding any appreciable blockage of the replenishment liquid flow even in narrow gaps. Critical heat flux (CHF) was found to generally increase with decreasing boiler gap. CHF for flat, micro-channel (with 0.2 mm rectangular fins) and mini-channel (with 1.98 mm rectangular fins) surfaces was 4.5, 5.9, and 5.7 times greater than for pool boiling from a flat surface for corresponding gaps. A pressure drop model was formulated to predict coolant mass flow rate, boiling surface inlet and exit velocities, and pressure drop components throughout the loop. The model predictions illustrate the pumpless loop's selfsustaining and self-enhancing attributes, and relate CHF trends to those of the two-phase mixture acceleration along the boiling surface. Index Terms-Boiling, critical heat flux, high heat flux, microchannel, passive cooling, phase change, pumpless loop, pressure drop. NOMENCLATURE Fin width. Flow area associated with boiler gap adjacent to boiling surface. Planform area of boiling surface, .

A comprehensive flow regime map for microchannel flow boiling with quantitative transition criteria

“…The different experimental cases listed in Table 1 The proposed criterion for transition between confined and unconfined flow is compared in Figure 5 with available experimental observations from other studies in the literature for water [1,3,7,[25][26][27][28][29][30][31], dielectric liquids [32][33][34][35], and refrigerants [36]. Details of the fluid, geometry, mass flux, and heat flux of the data points used in this comparison are listed in Table 2.…”

Adaptive Cooling of Integrated Circuits Using Digital Microfluidics

“…Microchannel cooling loops designed today now have high cooling performance, large heat transfer coefficients, small channel volumes, and a small cooling inventory. A pumpless loop, for example, has been presented with micro-channel surfaces for cooling [12]. A closedloop two-phase microchannel cooling system based on electroosmotic pumping of liquids has been developed at Stanford [11], and a MEMS-based micro capillary pumped loop (micro-CPL) has been fabricated on a silicon wafer where an evaporator, condenser, reservoir, and liquid lines are all integrated [18].…”

Effects of two-phase inlet quality, mass velocity, flow orientation, and heating perimeter on flow boiling in a rectangular channel: Part 2 – CHF experimental results and model

“…However, thermosyphons are associated with rather low critical heat flux (CHF) values, and often rely on additional surface augmentation techniques [3,4] to enhance boiling performance. Another method that relies on gravity to achieve passive fluid circulation and improve CHF is the pumpless loop concept [5] illustrated in Fig. 1(b).…”