A Review on efficient thermal management of air- and liquid-cooled data centers: From chip to the cooling system

Macro-to-microchannel transition in two-phase flow: Part 2 – Flow boiling heat transfer and critical heat flux

Cooling of server electronics: A design review of existing technology

This paper presents a synthesized review on the recent literature concerning micro-scale flow boiling. The topics covered are macro- to micro-scale transition, flow patterns, pressure drop, heat transfer coefficient, critical heat flux, superficial void fraction and liquid entrainment. The analyses revealed some characteristics common to micro-scale two-phase flow, i.e. absence of stratified flow, predominance of annular flow over all saturated region, uniform liquid film thickness during horizontal flows, reduced liquid entrainment, high heat transfer coefficients and pressure drops. Despite the importance of liquid entrainment and void fraction in predictive methods, there is a lack of experimental results for these parameters in the micro-scale literature. Important accomplishments concerning the investigation of micro-scale flow boiling have been obtained over the last two decades, but some aspects, including local physical mechanisms related to heat transfer, onset of dryout and flow boiling instabilities still remain unclear.

Flow boiling in micro-scale channels – Synthesized literature review

Fundamental issues, mechanisms and models of flow boiling heat transfer in microscale channels

A new flow pattern-based prediction method for heat transfer coefficients in microchannels was developed based on recent experimental results for several multi-microchannel evaporators in silicon and copper and for single-microchannel tubes in stainless steel. In the present paper, some updates to the three-zone flow boiling model for slug flow are presented, including further proof that the dryout thickness is well represented by setting it equal to the measured channel roughness for the silicon, copper and stainless steel test surfaces. Next, a non-circular channel version of the Cioncolini–Thome unified annular flow model for convective boiling is proposed. These two methods are joined together into a flow pattern-based method using a new heat flux-dependent flow pattern transition criterion between slug flow and annular flow. The method predicts the results quite accurately and also captures the trends in the heat transfer coefficients well.

Flow pattern-based flow boiling heat transfer model for microchannels

Two-phase flow of refrigerants in 85 μm-wide multi-microchannels: Part II ― Heat transfer with 35 local heaters

Two-phase flow of refrigerants in 85 μm-wide multi-microchannels: Part I – Pressure drop

This article presents new experimental results for two-phase flow boiling of R-134a, R-1234ze(E) and R-245fa in a micro-evaporator. The test section was made of copper and composed of 52 microchannels 163μm wide and 1560μm high with the channels separated by 178μm wide fins. The channels were 13.2mm long. There were 35 local heaters and temperature measurements arranged in a 5×7 array as a pseudo-CPU. The total pressure drops of the test section were below 20kPa in all cases. The wall heat transfer coefficients were generally above 10’000W/mK and a function of the heat flux, vapor quality and mass flux. A new flow pattern-based prediction method for flow boiling heat transfer coefficients in microchannels was developed based on the experimental results. The new prediction method also predicted published data for four other test sections accurately, capturing the trends versus vapor quality well.

Heat transfer charcacteristics in a copper micro-evaporator and flow pattern-based prediction method for flow boiling in microchannels

Thermal management of servers is becoming more and more challenging and current air-cooled systems are reaching their limits regarding cooling capacity. Power consumption of air-cooled data centers, now being monitored by the US Environmental Agency [1], is rising rapidly. Moving towards liquid-based cooling would decrease the power consumption and improve the cooling system's efficiency.

Etienne Costa-Patry

Papers

Flow pattern-based flow boiling heat transfer model for microchannels

Two-phase flow of refrigerants in 85 μm-wide multi-microchannels: Part II ― Heat transfer with 35 local heaters

Two-phase flow of refrigerants in 85 μm-wide multi-microchannels: Part I – Pressure drop

Heat transfer charcacteristics in a copper micro-evaporator and flow pattern-based prediction method for flow boiling in microchannels

Hot-spot self-cooling effects on two-phase flow of R245fa in 85µm-wide multi-microchannels