Experimental Investigation on Observed Scattering in Heat Transfer Characteristics for Flow Boiling in a Small Diameter Tube

Ohta, Hiroyuki; Inoue, Koichi; Ando, Makiko; Watanabe, Kei

doi:10.1080/01457630802290080

Cited by 21 publications

(9 citation statements)

References 14 publications

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“…However, the HTC still displays minima in the axial direction. This behavior has been observed in other investigations of boiling heat transfer in annular flow [13,[37][38][39] this trend as a tradeoff between partial dryout and rewetting phenomenon accompanied with the transition from slug/churn flow to annular flow [13,37,39] or flow instability/reversal [13,37], and thinning of liquid film thickness in the annular flow regime [13,37,39]. The issue of flow instability/ reversal can be excluded for the small amplitude of temperature fluctuations measured in our study of steady annular flow.…”

Section: Boiling Heat Transfer Of Untreated Hydrophilic Surfacesupporting

confidence: 86%

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Flow boiling in vertical narrow microchannels of different surface wettability characteristics

Zhou

Coyle

et al. 2017

International Journal of Heat and Mass Transfer

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An experimental investigation of saturated flow boiling in a high-aspect-ratio, one-sided heating rectangular microchannel was conducted with deionized water as the working fluid. The bare silicon wafer bottom surface of the microchannel was hydrophilic with a contact angle of 65° ± 3°, compared with the super-hydrophilic surface deposited by a thin film of 100-nm-thickness silicon dioxide through PECVD with a contact angle less than 5°. In experimental runs the mass fluxes were in the range of 120 kg/m 2 s-360 kg/m 2 s, the wall heat fluxes were spanned from 4 W/cm 2 to 20 W/cm 2 and the inlet vapor qualities were varied from 0.03 to 0.1. Parametric study and flow visualization on pressure drop, local heat transfer coefficient, and flow pattern for surfaces of different surface wettability characteristics were carried out. Measured total pressure drops in single phase and two phase flow experiments agreed well with predicted values. The experimental data points were almost all located in the annular flow regime, and the local heat transfer coefficients approached a constant value and then increased towards the exit along the flow direction. According to flow visualization, the local dryout phenomenon occurred on the untreated hydrophilic surface at high heat fluxes for low mass fluxes, accompanied with deteriorative heat transfer performance, while it was not observed on the super-hydrophilic surface at the identical condition. Meanwhile severe heat transfer deterioration was obtained on the hydrophilic surface with increased inlet vapor quality, while the heat transfer coefficient of the super-hydrophilic surface was relatively constant which outperformed the untreated silicon wafer surface without increased pressure drop penalty.

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Section: Boiling Heat Transfer Of Untreated Hydrophilic Surfacesupporting

confidence: 86%

“…This caused the initial drop in HTC. However, gradual evaporation and thinning of the liquid film accompanied with augmented vapor velocities caused the local HTC to rise [13,37,39], thus resulting in the minima in the middle of the test section. As shown in Fig.…”

Section: Boiling Heat Transfer Of Untreated Hydrophilic Surfacementioning

confidence: 99%

Flow boiling in vertical narrow microchannels of different surface wettability characteristics

Zhou

Coyle

et al. 2017

International Journal of Heat and Mass Transfer

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“…Additionally, according to [29], these liquid front waves seem to explain the fact that the heat transfer coefficient keeps increasing at vapor qualities of almost one due to the internal surface rewetting. In order to investigate the effect of thermal instabilities on the local heat transfer coefficient, Ohta et al [25] performed heat transfer coefficient measurements for the same tube under strict and weak inlet flow rate regulation. They observed that under weak regulations, wall temperature fluctuations start at much lower vapor qualities than for strict inlet flow rate regulation.…”

Section: Experimental Studymentioning

confidence: 99%

A Critical Overview on the Recent Literature Concerning Flow Boiling and Two-Phase Flows Inside Micro-Scale Channels

Ribatski

2013

Experimental Heat Transfer

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Studies on two-phase flow and flow boiling inside micro-scale channels published approximately until the middle of last decade were characterized by drastic discrepancies among authors. Frequently, differences of about one order of magnitude were observed for experimental results obtained at almost similar conditions. Parcels of these differences were related to the use of inaccurate experimental techniques and also for not taking into account all the relevant experimental parameters, such as surface roughness, presence of thermal instabilities, and flash gas effects. Therefore, it was not surprising that the proposed models and correlations worked reasonably well for restricted databases, mainly those considered during their development. Transitions between micro-and macro-scale were proposed based only on channel dimensions and manufacturing processes without taking into account heat transfer and two-phase flow mechanistic aspects. During this early period, most authors were just theorizing about the relevant physical mechanisms, and nucleate boiling was frequently proposed as the main heat transfer mechanism even at high vapor qualities under annular flow conditions. Most recently, as result of the large number of studies performed on this theme and the efforts of leading laboratories over the world, agreements between independent databases are observed and recent correlations and models are providing accurate predictions. The experimental techniques used by each laboratory were improved, resulting in more accurate results that have contributed to the actual status quo. Flow pattern predictive methods based on objective recognition techniques have been proposed. Researchers are suggesting that the transition between micro-and macro-behaviors may be due to the results of bubble confinement within the channel as well as the balance among gravitational, inertial, wetting, and surface tension forces. In addition, recent physical models have been proposed and their hypothesis investigated through numerical simulations and experimental studies focusing on determining the two-phase topology and temperature and velocity fields. Additionally, the knowledge of the mechanisms acting on the flow thermal instabilities has immerged as a critical aspect in order to predict the hydro-thermal performance of flow boiling in micro-scale channels. In this context, the present study concerns an overview on the recent literature (from 2005 to 2012) dealing with flow boiling and two-phase flow in micro-scale channels and was written having the main goal of identifying points of agreements among authors and aspects that remain unclear. NOMENCLATUREBo Bond number (dimensionless) Co confinement number (dimensionless) D diameter (m) D c characteristic diameter (m) D eq equivalent diameter (m) D h hydraulic diameter (m) D t r transitional diameter (m) g gravitational acceleration (m/s 2 ) G mass velocity (kg/m 2 s) h heat transfer coefficient (kg/m 2 s) H channel height (m) L Laplace constant (m); channel length (m) p pressure (Pa) p r r...

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“…However, because of uncertainties that still exist in the fundamental understanding of key phenomena underlying heat transfer and fluid flow at the micoscale, there are currently no commercially available devices that exploit this technology. Uncertainties include the dominant heat transfer mechanisms [2][3][4][5][6][7] the effect of channel aspect ratio on heat transfer rates and flow patterns, the effect of channel length, the effect of surface roughness [8][9][10] and flow instabilities [11,12]. Karayiannis et al [9] highlighted discrepancies in the findings of a number of studies in the literature leading to uncertainty in the correlations predicting the heat transfer coefficient and pressure drops.…”

Section: Introductionmentioning

confidence: 99%

Experimental Study of Flow Boiling Using R134a in Multi Microchannels

Al-Janabi¹,

Coletti²,

Mahmoud³

et al. 2019

Proceedings of the 5th World Congress on Mechanical, Chemical, and Material Engineering

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This paper presents the results of an experimental investigation of flow boiling heat transfer in multi microchannel heat sink. The test section, 20 mm long and 15 mm wide, consisted of 26 rectangular microchannels (350 µm wide, 630 µm deep-i.e. Dh = 450 µm-and 200 µm wall thickness) produced by CNC machining from an oxygen free copper block. Using R134a as a test fluid, the experiments were conducted at a system pressure of 7 bar, heat flux range 7.6-625.8 kW/m 2 and mass flux range 50-300 kg/m 2 s. A high speed camera was used to capture the flow patterns. Four flow patterns were observed namely bubbly, slug flow/confined bubble, churn flow and annular flow as the heat flux gradually increased. Results show that the maximum heat transfer coefficient obtained is 35.52 kW/m 2 K.The results also show that the heat transfer coefficient increased with heat flux and there was no mass flux effect. Relevant literature correlations predicting the heat transfer coefficient were evaluated against the current data.

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Experimental Investigation on Observed Scattering in Heat Transfer Characteristics for Flow Boiling in a Small Diameter Tube

Cited by 21 publications

References 14 publications

Flow boiling in vertical narrow microchannels of different surface wettability characteristics

Flow boiling in vertical narrow microchannels of different surface wettability characteristics

A Critical Overview on the Recent Literature Concerning Flow Boiling and Two-Phase Flows Inside Micro-Scale Channels

Experimental Study of Flow Boiling Using R134a in Multi Microchannels

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