Fundamental Issues, Technology Development, and Challenges of Boiling Heat Transfer, Critical Heat Flux, and Two-Phase Flow Phenomena with Nanofluids

Boiling heat transfer and corresponding two-phase flow phenomena are of significant interest for the design of a compact evaporator. The present work investigates experimentally, using a high-speed digital CCD camera, the two-phase flow phenomena for boiling in a silicon-based, two parallel trapezoid microchannels, which were prepared by the combination of silicon bulk micro machining and Pyrex-silicon wafer bonding. Onset of nucleate boiling, bubbly flow, slug flow, and partial dry out slug flow are typically observed along the flow direction. The appearance of the partial dryout slug flow may degrade the nucleate boiling heat transfer in the microchannel. At a low flow rate, reversed vapor flow is observed. In such a flow pattern, liquid droplets are formed intermittently on the inner wall of top Pyrex glass due to vapor condensation. Moreover, the reversed vapor flow usually accompanies with large magnitude two-phase flow oscillations.

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The effect of surfactant concentrations and surface material on heat transfer coefficient in nucleate boiling regime

Kerntechnik ◽

10.1515/kern-2020-0064 ◽

2021 ◽

Vol 86 (5) ◽

pp. 365-374

Author(s):

A. M. Refaey ◽

S. Elnaggar ◽

S. H. Abdel-Latif ◽

A. Hamza

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Heat Transfer Coefficient ◽

Transfer Coefficient ◽

Boiling Heat Transfer ◽

Two Phase Flow ◽

Surfactant Solution ◽

Nucleate Boiling ◽

Phase Flow ◽

Two Phase

Abstract The nucleate boiling regime and two-phase flow are greater importance to the safety analysis of nuclear reactors. In this study, the boiling heat transfer in nuclear reactor is numerical investigated. The computational fluid dynamics (CFD) code, ANSYS Fluent 17.2 is used and the boiling model is employed. The numerical predictions obtained are compared with the experimental data reported by A. Hamza et al. [9]. An experimental test rig is designed and constructed to investigate the effect of cooling water chemistry control and the material of heater surface. CFD software, allows the detailed analysis of the two-phase flow and heat transfer. In this paper, we evaluate the accuracy of the boiling model implemented in the ANSYS Fluent code. This model is based on the heat flux partitioning approach and accommodates the heat flux due to single-phase convection, quenching and evaporation. The validation carried out of surfactant fluid/vapor two-phase flow inside the 2-D cylindrical boiling vessel. A heated horizontal pipe with stainless steel, Aluminum, and Zircalloy surface materials are used to numerically predict the field temperature and void fraction. Different surfactant concentrations ranging from 0, (pure water) to 1500 ppm, and heat fluxes ranging from 31 to 110 kW/m2 are used. The results of the predicted model depict that the addition of SDS Surfactant and increasing the heat flux improves the coefficient of boiling heat transfer for a given concentration. Also, it was found that the increasing of the concentration of aqueous surfactant solution increases the pool boiling heat transfer coefficient. The aqueous surfactant solution SDS improved the heat transfer coefficient of Aluminum, Zircalloy and stainless steel surface materials by 135%.138% and 120% respectively. The results of the numerical model are nearly in agreement with that measured in experimental.

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Investigation of the critical heat flux in small-diameter channels

Nuclear Energy and Technology ◽

10.3897/nucet.7.65754 ◽

2021 ◽

Vol 7 (1) ◽

pp. 73-78

Author(s):

Vladimir I. Belozerov ◽

Aleksandr S. Gorbach

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Heat Exchange ◽

Critical Heat Flux ◽

Heat Exchangers ◽

Two Phase Flow ◽

Small Diameter ◽

Phase Flow ◽

Two Phase ◽

Channel Size

The paper describes experimental studies into the hydrodynamics and heat exchange in a forced water flow in small-diameter channels at low pressures. The timeliness of the studies has been defined by the growing interest in small-size heat exchangers. Small-diameter channels are actively used in components of compact heat exchangers for present-day engineering development applications. The major difficulty involved in investigation of heat-transfer processes in small-diameter channels consists in the absence of common methodologies to calculate coefficients of hydraulic resistance and heat transfer in a two-phase flow. The channel size influences the heat exchange and hydrodynamics of a two-phase flow as one of the determining parameters since the existing internal scales (vapor bubble size, liquid droplet diameter, film thickness) may become commensurable with the channel diameter, this leading potentially to different flow conditions. It is evident that one cannot justifiably expect a change in the momentum and energy transfer regularities in single-phase flows as the channel size is reduced for as long as the continuum approximation remains valid. The authors have analyzed the experiments undertaken by Russian scientists to investigate the distribution of thermal-hydraulic parameters in channels with a small cross-section in the entire variation range of the flow parameters in the channel up to the critical heat flux conditions when the wall temperature increases sharply as the thermal load grows slowly. The experimental critical heat flux data obtained by Russian and foreign authors has been compared.

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Experimental Study of Flow Boiling in Microchannel

ASME 5th International Conference on Nanochannels, Microchannels, and Minichannels ◽

10.1115/icnmm2007-30130 ◽

2007 ◽

Author(s):

S. G. Singh ◽

S. P. Duttagupta ◽

A. M. Kulkarni ◽

B. P. Puranik ◽

A. Agrawal

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Pressure Drop ◽

Flow Rate ◽

Heat Input ◽

Boiling Heat Transfer ◽

Two Phase Flow ◽

Phase Region ◽

Phase Flow ◽

Two Phase

With the reduction in size of electronic devices, the problem of efficient cooling is becoming more and more severe. Boiling heat transfer in microchannels is fast emerging as a promising solution to the problem. In the present work, microchannels were fabricated on a silicon wafer. A chrome-gold micro-heater was integrated and characterized on the other side of the wafer. The change in resistance of the micro-heater in the temperature range of 20 °C – 120 °C was found to be within 10%. Deionized water was used as working fluid in microchannel. The single-phase pressure drop across the microchannel was found to increase linearly with increasing flow rate in confirmation with conventional laminar flow theory. Also, the pressure drop decreases with an increase in heat input due to a reduction in viscosity. The study was extended to two phase flow with flow rate and heat flux as the control parameters. The onset of two phase flow, at a given heat flux, with a decrease in flow rate, can be identified by the departure of linear pressure drop to non-linearity; this point was also confirmed through visual observation. In two-phase region of flow, pressure drop was found to increase initially, passes through a maximum and then decreases, with a decrease in flow rate. The experiments are performed for several heat fluxes. Both the onset of two phase and maximum pressure drop in the two phase region shifts to higher flow rates with an increase in heat input. Such detailed experimental results seem to be missing from the literature and are expected to be useful for modeling of boiling heat transfer in microchannels. Another pertinent observation is presence of instability in two-phase flow. It was found that at higher flow rate and heat flux instability in two-phase flow was more. An attempt to record these instabilities was made and preliminary data on their frequency will be presented. This study may help to choose suitable operating conditions for a microchannel heat sink for use in electronics cooling.

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Study on Flow Boiling Heat Transfer and Two-Phase Flow Pressure Drop in Flat Mini-Channel

Volume 4: Codes, Standards, Licensing and Regulatory Issues; Student Paper Competition ◽

10.1115/icone17-75248 ◽

2009 ◽

Cited By ~ 1

Author(s):

Yasuo Koizumi ◽

Hiroyasu Ohtake ◽

Ken Sato

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Pressure Drop ◽

Cross Section ◽

Boiling Heat Transfer ◽

Two Phase Flow ◽

Phase Flow ◽

Two Phase ◽

Wide Cross ◽

Flow Pressure

Flow and the heat transfer characteristics of boiling two-phase flow of water in flat mini-rectangular-channels were examined. The cross-sections tested were 1.0×10 to 0.2×10 mm and the flow channel length was 250 mm. Single phase flow pressure drop was well expressed by the method for the usual size in the present experimental range. Boiling heat transfer of 0.5 mm high and 10 mm wide cross section was similar to that of the usual size. However, that 0.2 mm high and 10 mm wide cross section was a little different from that of the usual size. An increase in the heat flux after the onset of nucleate boiling on the boiling curve is milder than that of the usual size. Thus, the critical heat flux was lower than that of the usual size. Flow patterns observed in the present experiments were a little different from the Baker flow pattern chart. Consistent agreement was not obtained between the present results of the two-phase flow pressure drop and predictions by the methods for the usual size and also for a mini tube. Subcooled boiling was observed widely in the test section. This made it difficult to determine the local conditions such as quality that was necessary to calculate the Lockhart-Martinelli parameter for the two-phase flow pressure drop prediction.

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