Critical heat flux | EPFL Graph Search

In the study of heat transfer, critical heat flux (CHF) is the heat flux at which boiling ceases to be an effective form of transferring heat from a solid surface to a liquid. Description Boiling systems are those in which liquid coolant absorbs energy from a heated solid surface and undergoes a change in phase. In flow boiling systems, the saturated fluid progresses through a series of flow regimes as vapor quality is increased. In systems that utilize boiling, the heat transfer rate is significantly higher than if the fluid were a single phase (i.e. all liquid or all vapor). The more efficient heat transfer from the heated surface is due to heat of vaporization and sensible heat. Therefore, boiling heat transfer has played an important role in industrial heat transfer processes such as macroscopic heat transfer exchangers in nuclear and fossil power plants, and in microscopic heat transfer devices such as heat pipes and microchannels for cooling electronic chips. The u

Saturated flow boiling heat transfer and critical heat flux in small horizontal flattened tubes

Cristiano Bigonha Tibiriça, John Richard Thome

This paper presents experimental results for flow boiling heat transfer coefficient and critical heat flux (CHF) in small flattened tubes. The tested flattened tubes have the same equivalent internal diameter of 2.2 mm, but different aspect height/width ratios (H/W) of 1/4, 1/2, 2 and 4. The experimental data were compared against results for circular tubes using R134a and R245fa as working fluids at a nominal saturation temperature of 31 degrees C. For mass velocities higher than 200 kg/m(2)s, the flattened and circular tubes presented similar heat transfer coefficients. Such a behavior is related to the fact that stratification effects are negligible under conditions of higher mass velocities. Heat transfer correlations from the literature, usually developed using only circular-channel experimental data, predicted the flattened tube results for mass velocities higher than 200 kg/m(2)s with mean absolute error lower than 20% using the equivalent diameter to account for the geometry effect. Similarly, the critical heat flux results were found to be independent of the tube aspect ratio when the same equivalent length was kept. Equivalent length is a new parameter which takes into account the channel heat transfer area. The CHF correlations for round tubes predicted the flattened tube data relatively well when using the equivalent diameter and length. Furthermore, a new proposed CHF correlation predicted the present flattened tube data with a mean absolute error of 5%. (C) 2012 Elsevier Ltd. All rights reserved.

Elsevier2012

Flow Boiling Characteristics for R1234ze(E) in 1.0 and 2.2 mm Circular Channels

Cristiano Bigonha Tibiriça, John Richard Thome

Experimental flow boiling heat transfer results are presented for horizontal 1.0 and 2.2 mm I. D. (internal diameter) stainless steel tubes for tests with R1234ze(E), a new refrigerant developed as a substitute for R134a with a much lower global warming potential (GWP). The experiments were performed for these two tube diameters in order to investigate a possible transition between macro and microscale flow boiling behavior. The experimental campaign includes mass velocities ranging from 50 to 1500 kg/m(2) s, heat fluxes from 10 to 300 kW/m(2), exit saturation temperatures of 25, 31 and 35 degrees C, vapor qualities from 0.05 to 0.99 and heated lengths of 180 mm and 361 mm. Flow pattern characterization was performed using high speed videos. Heat transfer coefficient, critical heat flux and flow pattern data were obtained. R1234ze(E) demonstrated similar thermal performance to R134a data when running at similar conditions. [DOI: 10.1115/1.4004933]

2012

Experimental two-phase fluid flow in microchannels

Rémi Revellin

Micro or mini heat spreaders are used in the interest of providing higher cooling capability for microtechnologies. Heat spreaders using micro or mini channels are not yet well studied, for this the fundamentals of two-phase heat transfer in microchannels are being studied. Here, a comprehensive experimental two-phase flow study has been carried out on two single round tubes (D = 0.509 and 0.790 mm) and for two different fluids: R-134a and R-245fa. An optical measurement method for two-phase flow characterization in microtubes has been applied to determine the frequency of bubbles exiting a microevaporator, the coalescence rates of these bubbles and their lengths as well as their mean two-phase vapor velocity. Four principal flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions (bubbly/slug flow and slug/semi-annular flow) were observed. A new type of flow pattern map for evaporating flow in microchannel has been developed. The first zone corresponds to the isolated bubble regime. It includes both bubbly flow or/and slug flow and is present up to the onset of coalescence. The second zone is the coalescing bubble regime. It is present up to the end of coalescence process. The third zone is the annular zone and is limited by the fourth zone of this diabatic map, the onset of critical heat flux. This flow pattern map can be used for heat transfer model and design of micro evaporator. The vapor velocity or cross sectional void fraction have been measured. For R-134a, the flow can be considered to be homogeneous (or near homogeneous). For R-245fa, more tests exhibit instabilities and surprisingly show vapor velocities below those of homogeneous flow. Frictional two-phase pressure drops have been measured over a wide range of conditions for the two microchannels and two fluids. Three regimes are distinguishable when regarding to the variation of the adiabatic frictional pressure drop with the vapor quality or the two-phase friction factor with the two-phase Reynolds number: a laminar regime for ReTP < 2000, a transition regime for 2000 ≤ ReTP ≤ 8000 and a turbulent regime for ReTP ≥ 8000. The turbulent two-phase flows are best predicted by the Müller-Steinhagen correlation. New accurate CHF data have been measured with the test facility. A new microchannel version of the Katto-Ohno correlation has been developed to predict the CHF in circular, uniformly heated microchannels. Moreover, a new transition curve from annular flow to dryout has been proposed.

EPFL2005

Experimental two-phase fluid flow in microchannels

Rémi Revellin

EPFL2005