Cooling of microprocessors using flow boiling of CO2 in a micro-evaporator: Preliminary analysis and performance comparison

The flow pattern based flow boiling heat transfer and two-phase pressure drop models for CO2, recently developed by Cheng et al. [L Cheng, G. Ribatski. J. Moreno Quiben, J.R. Thome, New prediction methods for CO2 evaporation inside tubes: Part I - A two-phase flow pattern map and a flow pattern based phenomenological model for two-phase flow frictional pressure drops, Int. J. Heat Mass transfer 51 (2008) 111-124; L Cheng, G. Ribatski, J.R. Thome, New prediction methods for CO2 evaporation inside tubes: Part 11 - An updated general flow boiling heat transfer model based on flow patterns, Int. J. Heat Mass transfer 51 (2008) 125-135], have been used to predict the thermal performance Of CO2 in a silicon multi-microchannel evaporator (67 parallel channels with a width of 0.223 mm, a height of 0.68 mm and a length of 20 mm) for cooling of a microprocessor. First, some simulation results Of CO2 flow boiling heat transfer and two-phase pressure drops in microscale channels are presented. The effects of channel diameter, mass flux, saturation temperature and heat flux on flow boiling heat transfer coefficients and two-phase pressure drops are next addressed. Then, simulations of the base temperatures of the silicon multi-microchannel evaporator using R236fa and CO2 were performed for the following conditions: base heat fluxes from 20 to 100 W/cm(2), a mass flux of 987.6 kg/m(2)s and a saturation temperature of 25 degrees C. These show that the base temperatures using CO2 are much lower than those using R236fa. Compared to R236fa, CO2 has much higher heat transfer coefficients and lower pressure drops in the multi-microchannel evaporator. However, the operation pressure of CO2 is much higher than that of R236fa. Based on the analysis and comparison, CO2 appears to be a promising coolant for microprocessors at low operating temperatures but also presents a great technological challenge like other new cooling technologies. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.

Two-Phase Heat Transfer Mechanisms within Plate Heat Exchangers

Raffaele Luca Amalfi

Plate heat exchangers are adopted in many domestic and industrial applications, such as ventilation, air conditioning, evaporation or condensation process, heat pumps and cooling of hydrodynamic circuits in engines. In the present thesis, an extensive experimental study to characterize thermal and hydraulic performance of a compact plate heat exchanger has been carried out and then follow up with proposition of new prediction methods for single-phase and evaporating flows in the plate¿s channels, in the form of general correlations that include large additional databases culled from the literature. Specifically, upward single-phase flow and flow boiling heat transfer of low pressure liquid refrigerants were investigated within a plate heat exchanger prototype fabricated with 1 mm pressing depth and chevron angle of 65°. High spatial and temporal resolution infrared measurements were implemented to obtain local (pixel-by-pixel) heat transfer coefficients and frictional pressure drops. Single-phase experiments were carried out for Reynolds numbers ranging from 34 to 1615 and Prandtl numbers from 4.9 until 6.5, and the associated effect of the main involved parameters on the thermal and hydraulic performance was analyzed in detail. Several of the most quoted prediction methods available in the literature were statistically evaluated against the present heat transfer and pressure drop database and a new models were proposed to predict mean and local thermal and hydraulic performance of the present compact plate heat exchanger. Two-phase experiments were carried out for mass fluxes ranging from 10 kgm-2s-1 to 85 kgm-2s-1, imposed heat fluxes from 225 Wm-2 to 4100 Wm-2, saturation temperatures from 19°C to 35°C and vapor qualities from 0.05 to 0.90. The heat transfer coefficient increased with mass flux, heat flux and saturation temperature (system pressure) whilst the frictional pressure drop rose with mass flux and vapor quality but decreased with saturation temperature. Based on local infrared measurements, a new correlation for predicting local frictional pressure gradient through the test section was proposed. Next, a wide experimental databank was culled from thirteen literature research studies, which investigated flow boiling heat transfer and two-phase frictional pressure drop within chevron plate heat exchangers. The database was first adopted to validate the predicted capabilities of 29 literature models, and then utilized to provide the new prediction methods to evaluate local heat transfer coefficients and frictional pressure gradient. These new models were developed from 1903 heat transfer and 1513 frictional pressure drop data points (3416 total) and were proved to work better over a very wide range of operating conditions, plate designs and fluids (including ammonia). The general flow boiling prediction methods have been programmed into a simulation code to analyze local thermal and hydraulic performance of the plate heat exchangers over a large range of test conditions. The simulation results were then validated against independent experimental database from literature, resulting in very good agreement between each other. The present simulation code represents a powerful tool to be used for practical applications by the thermal engineers in order to design and rate commercial plate heat exchangers.

EPFL2016

Experimental study on flow boiling heat transfer of multiport tubes with R245fa and R1234ze(E)

Bruno Agostini, John Richard Thome, Farzad Vakili Farahani

The upward flow boiling experiments were carried out in a flat aluminum extruded multiport tube, which is composed of 7 parallel rectangular channels (1.1 mm × 2.1 mm) with hydraulic diameter of 1.4 mm. Two refrigerants, R245fa and R1234ze, the latter a recent environmentally safe refrigerant proposed as a potential replacement for R134a, were tested. A new hot water heating technique that accounts for either uniform or non-uniform local heat flux distribution along the channel was developed to obtain and reduce the data. Effects of heat flux, mass flux, vapor quality, and saturation temperature on flow boiling heat transfer in multiport tubes were considered. Finally, the experimental results were compared with some well-known correlations to evaluate the capabilities of existing prediction methods. The analysis shows that the three-zone model for slug flows works well for that subset of test results, utilizing the apparent surface roughness in place of the dryout thickness in the model, as has been previously done for silicon, stainless steel and copper microchannels with measured surface roughnesses.

Elsevier2013

EXPERIMENTAL STUDY ON FLOW BOILING HEAT TRANSFER OF MULTIPORT TUBES WITH R245fa AND R1234ze(E)

Bruno Agostini, John Richard Thome, Farzad Vakili Farahani

This article presents an experimental study of upward flow boiling heat transfer in multiport tubes. Usually, multiport tubes are utilized horizontally in automotive and air-conditioning systems while here there is interest in vertical flow as part of a two-phase thermosyphon electronics cooling system. Experiments were carried out in a flat aluminum extruded multiport tube, which was composed of 7 parallel rectangular channels (1.1mm × 2.1mm) with a hydraulic diameter of 1.4 mm. Two refrigerants, R245fa and the new refrigerant R1234ze, the latter a new environmentally safe refrigerant proposed as a potential replacement for R134a, were tested. A new hot water heating technique that accounts for either uniform or non-uniform local heat flux distribution along the channel, particularly for exploring high vapor qualities into the dryout region, was developed to obtain and reduce the data. Accordingly, heat transfer coefficients were measured locally for the entire range of vapor qualities starting from subcooled liquid to superheated vapor. Effects of heat flux, mass flux, vapor quality, and saturation temperature on flow boiling heat transfer in multiport tubes were considered. Finally, the experimental results were compared with some well-known correlations to evaluate the capabilities of existing prediction methods. The analysis completed so far shows that the three-zone model for slug flows works well for that subset of test results, utilizing the apparent surface roughness in place of the dryout thickness in the model, as has been previously done for silicon, stainless steel and copper microchannels with measured surface roughnesses. Notably, the non-dryout data were well predicted by the Cooper nucleate pool boiling correlation as in many other studies, although there is no proof that nucleate boiling is taking place except near the point of inception of boiling.

2012

Two-Phase Heat Transfer Mechanisms within Plate Heat Exchangers

Raffaele Luca Amalfi

EPFL2016

EXPERIMENTAL STUDY ON FLOW BOILING HEAT TRANSFER OF MULTIPORT TUBES WITH R245fa AND R1234ze(E)

Bruno Agostini, John Richard Thome, Farzad Vakili Farahani

2012